What is climate?
Climate in a narrow sense is usually defined as the "average weather," or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period is 30 years, as defined by the World Meteorological Organization (WMO). These quantities are most often surface variables such as temperature, precipitation, and wind. Climate in a wider sense is the state, including a statistical description, of the climate system.
What is the climate system?
The climate system is the highly complex system consisting of five major components: the atmosphere, the hydrosphere, the cryosphere, the land surface and the biosphere, and the interactions between them. The climate system evolves in time under the influence of its own internal dynamics and because of external forcings such as volcanic eruptions, solar variations and human-induced forcings such as the changing composition of the atmosphere and land-use change.
What factors determine earth's climate?
The climate system evolves in time under the influence of its own internal dynamics and due to changes in external factors that affect climate (called ‘forcings’). External forcings include natural phenomena such as volcanic eruptions and solar variations, as well as human-induced changes in atmospheric composition. Solar radiation powers the climate system. There are three fundamental ways to change the radiation balance of the Earth: 1) by changing the incoming solar radiation (e.g., by changes in Earth’s orbit or in the Sun itself); 2) by changing the fraction of solar radiation that is reflected (called ‘albedo’; e.g., by changes in cloud cover, atmospheric particles or vegetation); and 3) by altering the longwave radiation from Earth back towards space (e.g., by changing greenhouse gas concentrations). Climate, in turn, responds directly to such changes, as well as indirectly, through a variety of feedback mechanisms.
The amount of energy reaching the top of Earth’s atmosphere each second on a surface area of one square meter facing the Sun during daytime is about 1,370 Watts, and the amount of energy per square meter per second averaged over the entire planet is one-quarter of this (see Figure 1). About 30% of the sunlight that reaches the top of the atmosphere is reflected back to space. Roughly two-thirds of this reflectivity is due to clouds and small particles in the atmosphere known as ‘aerosols’. Light-colored areas of Earth’s surface – mainly snow, ice and deserts – reflect the remaining one-third of the sunlight. The most dramatic change in aerosol-produced reflectivity comes when major volcanic eruptions eject material very high into the atmosphere. Rain typically clears aerosols out of the atmosphere in a week or two, but when material from a violent volcanic eruption is projected far above the highest cloud, these aerosols typically influence the climate for about a year or two before falling into the troposphere and being carried to the surface by precipitation. Major volcanic eruptions can thus cause a drop in mean global surface temperature of about half a degree celsius that can last for months or even years. Some man-made aerosols also significantly reflect sunlight.
The energy that is not reflected back to space is absorbed by the Earth’s surface and atmosphere. This amount is approximately 240 Watts per square meter (W m–2). To balance the incoming energy, the Earth itself must radiate, on average, the same amount of energy back to space. The Earth does this by emitting outgoing longwave radiation. Everything on Earth emits longwave radiation continuously. That is the heat energy one feels radiating out from a fire; the warmer an object, the more heat energy it radiates. To emit 240 W m–2, a surface would have to have a temperature of around –19°C. This is much colder than the conditions that actually exist at the Earth’s surface (the global mean surface temperature is about 14°C). Instead, the necessary –19°C is found at an altitude about 5 km above the surface.
The reason the Earth’s surface is this warm is the presence of greenhouse gases, which act as a partial blanket for the longwave radiation coming from the surface. This blanketing is known as the natural greenhouse effect. The most important greenhouse gases are water vapor and carbon dioxide. The two most abundant constituents of the atmosphere – nitrogen and oxygen – have no such effect. Clouds, on the other hand, do exert a blanketing effect similar to that of the greenhouse gases; however, this effect is offset by their reflectivity, such that on average, clouds tend to have a cooling effect on climate (although locally one can feel the warming effect: cloudy nights tend to remain warmer than clear nights because the clouds radiate longwave energy back down to the surface). Human activities intensify the blanketing effect through the release of greenhouse gases. For instance, the amount of carbon dioxide in the atmosphere has increased by about 35% in the industrial era, and this increase is known to be due to human activities, primarily the combustion of fossil fuels and removal of forests. Thus, humankind has dramatically altered the chemical composition of the global atmosphere with substantial implications for climate.
Because the Earth is a sphere, more solar energy arrives for a given surface area in the tropics than at higher latitudes, where sunlight strikes the atmosphere at a lower angle. Energy is transported from the equatorial areas to higher latitudes via atmospheric and oceanic circulations, including storm systems. Energy is also required to evaporate water from the sea or land surface, and this energy, called latent heat, is released when water vapor condenses in clouds (see Figure 1). Atmospheric circulation is primarily driven by the release of this latent heat. Atmospheric circulation in turn drives much of the ocean circulation through the action of winds on the surface waters of the ocean, and through changes in the ocean’s surface temperature and salinity through precipitation and evaporation.
Due to the rotation of the Earth, the atmospheric circulation patterns tend to be more east-west than north-south. Embedded in the mid-latitude westerly winds are large-scale weather systems that act to transport heat toward the poles. These weather systems are the familiar migrating low- and high-pressure systems and their associated cold and warm fronts. Because of land-ocean temperature contrasts and obstacles such as mountain ranges and ice sheets, the circulation system’s planetary-scale atmospheric waves tend to be geographically anchored by continents and mountains although their amplitude can change with time. Because of the wave patterns, a particularly cold winter over North America may be associated with a particularly warm winter elsewhere in the hemisphere. Changes in various aspects of the climate system, such as the size of ice sheets, the type and distribution of vegetation or the temperature of the atmosphere or ocean will influence the large-scale circulation features of the atmosphere and oceans.
There are many feedback mechanisms in the climate system that can either amplify (‘positive feedback’) or diminish (‘negative feedback’) the effects of a change in climate forcing. For example, as rising concentrations of greenhouse gases warm Earth’s climate, snow and ice begin to melt. This melting reveals darker land and water surfaces that were beneath the snow and ice, and these darker surfaces absorb more of the Sun’s heat, causing more warming, which causes more melting, and so on, in a self-reinforcing cycle. This feedback loop, known as the ‘ice-albedo feedback’, amplifies the initial warming caused by rising levels of greenhouse gases. Detecting, understanding and accurately quantifying climate feedbacks have been the focus of a great deal of research by scientists unravelling the complexities of Earth’s climate.
What is climate change?
Climate change refers to a statistically significant variation in either the mean state of the climate or in its variability, persisting for an extended period (typically decades or longer). Climate change may be due to natural internal processes or external forcings, or to persistent anthropogenic changes in the composition of the atmosphere or in land use. Note that the United Nations Framework Convention on Climate Change (UNFCCC) defines "climate change" as: "a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods". The UNFCCC thus makes a distinction between "climate change" attributable to human activities altering the atmospheric composition, and "climate variability" attributable to natural causes.
What is the average temperature of the Earth now?
The analyses made by leading climate centers rank the year 2007 amongst the ten warmest years on record. According to the Met Office Hadley Centre in the UK, the global mean surface temperature in 2007 was 0.40°C (0.72°F) above the 1961–1990 annual average (14°C/57.2°F) and hence marks the seventh warmest year on record. According to the National Climatic Data Center of the National Oceanic and Atmospheric Administration, the global mean surface temperature anomaly was 0.55°C (0.99°F) above the twentieth century average (1901–2000) of 13.9°C (56.9°F), which ranks 2007 the fifth warmest year in its record. The unusual warmth in 2007 is noteworthy because it occurs at a time when solar irradiance is at a minimum and the equatorial Pacific Ocean is in the cool phase of its natural El Niño-La Niña cycle.
What is the greenhouse effect, and is it affecting our climate?
The greenhouse effect is unquestionably real and helps to regulate the temperature of our planet. It is essential for life on Earth and is one of Earth's natural processes. The solar energy absorbed by the Earth is ultimately released as thermal radiation emitted by the land and ocean. Some of this thermal radiation is absorbed by the atmosphere, including clouds, and reradiated back to Earth. This is called the greenhouse effect. The glass walls in a greenhouse reduce airflow and increase the temperature of the air inside. Analogously, but through a different physical process, the Earth’s greenhouse effect warms the surface of the planet. Water vapor is the most abundant greenhouse gas, followed by carbon dioxide and other trace gases. Without a natural greenhouse effect, the temperature of the Earth would be about zero degrees F (-18°C) instead of its present 57°F (14°C). So, the concern is not with the fact that we have a greenhouse effect, but whether human activities are leading to an enhancement of the greenhouse effect by the emission of greenhouse gases through fossil fuel combustion and deforestation. Human activities, primarily the burning of fossil fuels and clearing of forests, have greatly intensified the natural greenhouse effect, causing global warming.
Are greenhouse gases increasing?
Human activity has been increasing the concentration of greenhouse gases in the atmosphere (mostly carbon dioxide (CO2) from combustion of coal, oil, and gas; plus a few other trace gases). There is no scientific debate on this point. High precision measurements of atmospheric CO2 show that its average global concentration in 2006 was more than 381 parts per million by volume (ppmv), about 70 ppm higher than the first direct atmospheric measurements made in the 1950s. Records from Mauna Loa Observatory in Hawaii and from the South Pole show nearly the same rate of rise over time, demonstrating that the rise is global in extent. Measurements from dozens of sites around the world now confirm the overall rise. By comparison, pre-industrial levels of carbon dioxide (prior to the start of the Industrial Revolution) were about 280 ppmv. The global concentration of CO2 in our atmosphere today far exceeds the natural range over the last 650,000 years of 180 to 300 ppmv. According to the IPCC Special Report on Emission Scenarios (SRES), by the end of the 21st century, we could expect to see carbon dioxide concentrations of anywhere from 490 to 1260 ppm (75-350% above the pre-industrial concentration).
The record of CO2 at the Mauna Loa Observatory is known as the Keeling curve in honor of its originator, Charles David Keeling of Scripps Institution of Oceanography. The curve shows an annual seasonal cycle and a steady upward trend since measurements began at Mauna Loa in 1958. The seasonal cycle is due to the vast land mass of the Northern Hemisphere, which contains the majority of land-based vegetation. The result is a decrease in atmospheric carbon dioxide during northern spring and summer, when plants are absorbing CO2 as part of photosynthesis. The pattern reverses, with an increase in atmospheric carbon dioxide during northern fall and winter. The yearly spikes during the cold months occur as annual vegetation dies and leaves fall and decompose, which releases their carbon back into the air.
How do we know that the CO2 increase is caused by human activities?
Industry data provides detailed figures of fossil fuels used in various sectors. This data can be used to calculate the amount of CO2 released into the atmosphere by combustion of the fuels. The emissions are more than sufficient to explain the observed increase in atmospheric CO2 Careful analysis of the atmospheric CO2 data collected by Scripps and other organizations shows that CO2 is increasing at a rate that is about 44% slower than would be expected if all the CO2 from the burning of fossil fuels stayed in the air. The real puzzle is to explain where the missing 44% of the emissions have gone. The answer is that this "missing" CO2 is absorbed by both the oceans and the terrestrial biosphere. On average over the last 50 years the oceans and the terrestrial biosphere have continued to "mop up" this amount of CO2. Whether they will continue to do this as atmospheric CO2 concentrations continue to increase is a critical question and the subject of intense international research.
Other evidence for a human cause: 1) There are no known natural sources of CO2 sufficient to account for the recent increase. 2) There are no known sinks of CO2 sufficient to have absorbed all the CO2 from fossil-fuel burning. 3) For more than 10,000 years prior to the Industrial Revolution, atmospheric CO2 levels were essentially constant, which shows that the recent increase is not natural. 4) The increase in CO2 has been accompanied by a decrease in O2 and by changes in the ratios of the isotopes of carbon in the CO2. The O2 and isotopes changes indicate that the CO2 increase was derived from the oxidation of old organic matter – consistent with burning fossil fuel. 5) The pattern of CO2 increase since 1958 has closely mirrored that of fossil-fuel burning.
What caused the Ice Ages and other important climate changes before the Industrial Era?
Climate on Earth has changed on all time scales, including long before human activity could have played a role. Great progress has been made in understanding the causes and mechanisms of these climate changes. Changes in Earth’s radiation balance were the principal driver of past climate changes, but the causes of such changes are varied. For each case – be it the Ice Ages, the warmth at the time of the dinosaurs or the fluctuations of the past millennium – the specific causes must be established individually. In many cases, this can now be done with good confidence, and many past climate changes can be reproduced with quantitative models.
There are three fundamental ways the Earth’s radiation balance can change, thereby causing a climate change: (1) changing the incoming solar radiation (e.g., by changes in the Earth’s orbit or in the Sun itself), (2) changing the fraction of solar radiation that is reflected (this fraction is called the albedo – it can be changed, for example, by changes in cloud cover, small particles called aerosols or land cover), and (3) altering the longwave energy radiated back to space (e.g., by changes in greenhouse gas concentrations). In addition, local climate also depends on how heat is distributed by winds and ocean currents. All of these factors have played a role in past climate changes.
Is the current climate change unusual compared to earlier changes in Earth's history?
Climate has changed on all time scales throughout Earth’s history. Some aspects of the current climate change are not unusual, but others are. The concentration of CO2 in the atmosphere has reached a record high relative to more than the past half-million years, and has done so at an exceptionally fast rate. Current global temperatures are warmer than they have ever been during at least the past five centuries, probably even for more than a millennium. If warming continues unabated, the resulting climate change within this century would be extremely unusual in geological terms. Although large climate changes have occurred in the past, there is no evidence that these took place at a faster rate than present warming. If projections of approximately 5°C warming in this century (the upper end of the range) are realized, then the Earth will have experienced about the same amount of global mean warming as it did at the end of the last ice age; there is no evidence that this rate of possible future global change was matched by any comparable global temperature increase of the last 50 million years.
Another unusual aspect of recent climate change is its cause: past climate changes were natural in origin, whereas most of the warming of the past 50 years is attributable to human activities. Human activities, primarily the burning of fossil fuels and clearing of forests, have greatly intensified the natural greenhouse effect, causing global warming.
Can the warming of the 20th century be explained by natural variability?
It is very unlikely that the 20th-century warming can be explained by natural causes. The late 20th century has been unusually warm. Palaeoclimatic reconstructions show that the second half of the 20th century was likely the warmest 50-year period in the Northern Hemisphere in the last 1300 years. This rapid warming is consistent with the scientific understanding of how the climate should respond to a rapid increase in greenhouse gases like that which has occurred over the past century, and the warming is inconsistent with the scientific understanding of how the climate should respond to natural external factors such as variability in solar output and volcanic activity. Climate models provide a suitable tool to study the various influences on the Earth’s climate. When the effects of increasing levels of greenhouse gases are included in the models, as well as natural external factors, the models produce good simulations of the warming that has occurred over the past century. The models fail to reproduce the observed warming when run using only natural factors. When human factors are included, the models also simulate a geographic pattern of temperature change around the globe similar to that which has occurred in recent decades. This spatial pattern, which has features such as a greater warming at high northern latitudes, differs from the most important patterns of natural climate variability that are associated with internal climate processes, such as El Niño.
Although natural internal climate processes, such as El Niño, can cause variations in global mean temperature for relatively short periods, analysis indicates that a large portion is due to external factors such as volcanic activity and variations in solar output. Brief periods of global cooling have followed major volcanic eruptions, such as Mt. Pinatubo in 1991. In the early part of the 20th century, global average temperature rose, during which time greenhouse gas concentrations started to rise, solar output was probably increasing and there was little volcanic activity. During the 1950s and 1960s, average global temperatures levelled off, as increases in aerosols from fossil fuels and other sources cooled the planet. The eruption of Mt. Agung in 1963 also put large quantities of reflective dust into the upper atmosphere. The rapid warming observed since the 1970s has occurred in a period when the increase in greenhouse gases has dominated over all other factors.
Numerous experiments have been conducted using climate models to determine the likely causes of the 20th-century climate change. These experiments indicate that models cannot reproduce the rapid warming observed in recent decades when they only take into account variations in solar output and volcanic activity. However, as shown in Figure 5, models are able to simulate the observed 20th-century changes in temperature when they include all of the most important external factors, including human influences from sources such as greenhouse gases and natural external factors. The human influence on climate very likely dominates over all other causes of change in global average surface temperature during the past half century.
What is radiative forcing?
The influence of a factor that can cause climate change, such as a greenhouse gas, is often evaluated in terms of its radiative forcing. Radiative forcing is a measure of how the energy balance of the Earth-atmosphere system is influenced when factors that affect climate are altered. The word radiative arises because these factors change the balance between incoming solar radiation and outgoing infrared radiation within the Earth’s atmosphere. This radiative balance controls the Earth’s surface temperature. The term forcing is used to indicate that Earth’s radiative balance is being pushed away from its normal state. Radiative forcing is usually quantified as the ‘rate of energy change per unit area of the globe as measured at the top of the atmosphere’, and is expressed in units of ‘Watts per square meter.’ When radiative forcing from a factor or group of factors is evaluated as positive, the energy of the Earth-atmosphere system will ultimately increase, leading to a warming of the system. In contrast, for a negative radiative forcing, the energy will ultimately decrease, leading to a cooling of the system. Important challenges for climate scientists are to identify all the factors that affect climate and the mechanisms by which they exert a forcing, to quantify the radiative forcing of each factor and to evaluate the total radiative forcing from the group of factors.
What is "abrupt climate change"?
There is an abundance of scientific evidence that shows major and widespread climate changes have occurred with startling speed. For example, roughly half of the warming of the North Atlantic Ocean since the last ice age was achieved in only a decade, and this warming was accompanied by significant changes in climate across most of the globe. Research over the past decade has shown that these abrupt – or nonlinear – climate changes have been especially common when the climate system was being forced to change most rapidly. Thus, the rate of buildup of carbon dioxide in the atmosphere may increase the possibility of large, abrupt and unwelcome regional or global climate events.
The mechanisms of past abrupt climate changes are not yet fully understood, and climate models typically underestimate the size, speed and extent of those changes. Hence, future abrupt changes cannot be predicted with confidence. Yet, because of greenhouse warming and other human alterations of the earth system, and the long lifetime of carbon dioxide in the atmosphere, certain thresholds are likely to be crossed and we will not know we have crossed them until it is too late to alter the outcome.
What is global dimming and what relevance does it have to climate change?
Global dimming is the term used to describe the observations from surface instruments showing a general reduction in the amount of solar radiation reaching the ground since about 1960, globally amounting to 2–3% per decade, up to about 1990. The dimming is variable from place to place, with some sites even showing a brightening over the period, but greatest in northern mid-latitudes. However, more recent research indicates that this trend reversed in about 1990 and since then there has been some 'global brightening', although being indirectly measured from satellites these more recent estimates may be less robust. It seems likely that the reductions, and perhaps the recent increases, may be due to changes in aerosols such as sulphates and soot (black carbon). The most recent version of the Hadley Centre climate model (HadGEM1), which includes both sulphate aerosols and soot, does simulate a reduction in surface solar radiation, though not as great as that actually observed. Neither the observations nor the implications for predictions of climate change are yet clear, and this is a subject of active research.
How is precipitation changing?
Precipitation is the general term for rainfall, snowfall and other forms of frozen or liquid water falling from clouds. Observations show that changes are occurring in the amount, intensity, frequency and type of precipitation. These aspects of precipitation generally exhibit large natural variability, and El Niño and changes in atmospheric circulation patterns such as the North Atlantic Oscillation have a substantial influence. Pronounced long-term trends from 1900 to 2005 have been observed in precipitation amount in some places: significantly wetter in eastern North and South America, northern Europe and northern and central Asia, but drier in the Sahel, southern Africa, the Mediterranean and southern Asia. More precipitation now falls as rain rather than snow in northern regions. Widespread increases in heavy precipitation events have been observed, even in places where total amounts have decreased. These changes are associated with increased water vapor in the atmosphere arising from the warming of the world’s oceans, especially at lower latitudes. There are also increases in some regions in the occurrences of both droughts and floods.
Has there been a change in extreme events like heat waves, droughts, floods and hurricanes?
Yes; the type, frequency and intensity of extreme events are expected to change as Earth’s climate changes, and these changes could occur even with relatively small mean climate changes. Changes in some types of extreme events have already been observed, for example, increases in the frequency and intensity of heat waves and heavy precipitation events. Since 1950, the number of heat waves has increased and widespread increases have occurred in the numbers of warm nights. The extent of regions affected by droughts has also increased as precipitation over land has marginally decreased while evaporation has increased due to warmer conditions. Generally, numbers of heavy daily precipitation events that lead to flooding have increased, but not everywhere. Tropical storm and hurricane frequencies vary considerably from year to year, but evidence suggests substantial increases in intensity and duration since the 1970s. In the extratropics, variations in tracks and intensity of storms reflect variations in major features of the atmospheric circulation, such as the North Atlantic Oscillation.
In a warmer future climate, there will be an increased risk of more intense, more frequent and longer-lasting heat waves. A related aspect of temperature extremes is that there is likely to be a decrease in the daily (diurnal) temperature range in most regions. It is also likely that a warmer future climate would have fewer frost days (i.e., nights where the temperature dips below freezing). Growing season length is related to number of frost days, and has been projected to increase as climate warms. There is likely to be a decline in the frequency of cold air outbreaks (i.e., periods of extreme cold lasting from several days to over a week) in Northern Hemisphere winter in most areas. Exceptions could occur in areas with the smallest reductions of extreme cold in western North America, the North Atlantic and southern Europe and Asia due to atmospheric circulation changes.
In a warmer future climate, summer dryness and winter wetness will increase in most parts of the northern middle and high latitudes. Summer dryness indicates a greater risk of drought. Along with the risk of drying, there is an increased chance of intense precipitation and flooding due to the greater water-holding capacity of a warmer atmosphere.
In concert with the results for increased extremes of intense precipitation, there would be an increase in extreme rainfall intensity. In particular, over Northern Hemisphere land, an increase in the likelihood of very wet winters is projected over much of central and northern Europe due to the increase in intense precipitation during storm events, suggesting an increased chance of flooding over Europe and other mid-latitude regions due to more intense rainfall and snowfall events producing more runoff.
There is evidence from modeling studies that future tropical cyclones could become more severe, with greater wind speeds and more intense precipitation. Studies suggest that such changes may already be underway; there are indications that the average number of Category 4 and 5 hurricanes per year has increased over the past 30 years.
Is the amount of snow and ice on the Earth decreasing?
Yes. Observations show a global-scale decline of snow and ice over many years, especially since 1980 and increasing during the past decade, despite growth in some places and little change in others. Most mountain glaciers are getting smaller. Snow cover is retreating earlier in the spring. Sea ice in the Arctic is shrinking in all seasons, most dramatically in summer. Reductions are reported in permafrost, seasonally frozen ground and river and lake ice. Important coastal regions of the ice sheets on Greenland and West Antarctica, and the glaciers of the Antarctic Peninsula, are thinning and contributing to sea level rise. The total contribution of glacier, ice cap and ice sheet melt to sea level rise is estimated as 1.2 ± 0.4 mm yr–1 for the period 1993 to 2003. Continuous satellite measurements capture most of the Earth’s seasonal snow cover on land, and reveal that Northern Hemisphere spring snow cover has declined by about 2% per decade since 1966, although there is little change in autumn or early winter. In many places, the spring decrease has occurred despite increases in precipitation.
Since 1978, satellite data have provided continuous coverage of sea ice extent in both polar regions. For the Arctic, average annual sea ice extent has decreased by 2.7 ± 0.6% per decade, while summer sea ice extent has decreased by 7.4 ± 2.4% per decade. The antarctic sea ice extent exhibits no significant trend. Thickness data, especially from submarines, are available but restricted to the central Arctic, where they indicate thinning of approximately 40% between the period 1958 to 1977 and the 1990s. This is likely an overestimate of the thinning over the entire arctic region however.
Most mountain glaciers and ice caps have been shrinking, with the retreat probably having started about 1850. Although many Northern Hemisphere glaciers had a few years of near-balance around 1970, this was followed by increased shrinkage. Melting of glaciers and ice caps contributed 0.77 ± 0.22 mm yr–1 to sea level rise between 1991 and 2004.
Taken together, the ice sheets of Greenland and Antarctica are very likely shrinking, with Greenland contributing about 0.2 ± 0.1 mm yr–1 and Antarctica contributing 0.2 ± 0.35 mm yr–1 to sea level rise over the period 1993 to 2003. There is evidence of accelerated loss through 2005. Thickening of high-altitude, cold regions of Greenland and East Antarctica, perhaps from increased snowfall, has been more than offset by thinning in coastal regions of Greenland and West Antarctica in response to increased ice outflow and increased Greenland surface melting.
Is sea level rising?
Yes, there is strong evidence that global sea level gradually rose in the 20th century and is currently rising at an increased rate, after a period of little change between AD 0 and AD 1900. Sea level is projected to rise at an even greater rate in this century. The two major causes of global sea level rise are thermal expansion of the oceans (water expands as it warms) and the loss of land-based ice due to increased melting.
Global sea level rose by about 120 meters during the several millennia that followed the end of the last ice age (approximately 21,000 years ago), and stabilized between 3,000 and 2,000 years ago. Sea level indicators suggest that global sea level did not change significantly from then until the late 19th century. The instrumental record of modern sea level change shows evidence for onset of sea level rise during the 19th century. Estimates for the 20th century show that global average sea level rose at a rate of about 1.7 mm yr–1.
Satellite observations available since the early 1990s provide more accurate sea level data with nearly global coverage. This decade-long satellite altimetry data set shows that since 1993, sea level has been rising at a rate of around 3 mm yr–1, significantly higher than the average during the previous half century. Coastal tide gauge measurements confirm this observation, and indicate that similar rates have occurred in some earlier decades.
In agreement with climate models, satellite data and hydrographic observations show that sea level is not rising uniformly around the world. In some regions, rates are up to several times the global mean rise, while in other regions sea level is falling. Substantial spatial variation in rates of sea level change is also inferred from hydrographic observations. Spatial variability of the rates of sea level rise is mostly due to non-uniform changes in temperature and salinity and related to changes in the ocean circulation.
Near-global ocean temperature data sets made available in recent years allow a direct calculation of thermal expansion. It is believed that on average, over the period from 1961 to 2003, thermal expansion contributed about one-quarter of the observed sea level rise, while melting of land ice accounted for less than half. Thus, the full magnitude of the observed sea level rise during that period was not satisfactorily explained by those data sets, as reported in the IPCC Third Assessment Report.
During recent years (1993–2003), for which the observing system is much better, thermal expansion and melting of land ice each account for about half of the observed sea level rise, although there is some uncertainty in the estimates.
The reasonable agreement in recent years between the observed rate of sea level rise and the sum of thermal expansion and loss of land ice suggests an upper limit for the magnitude of change in land-based water storage, which is relatively poorly known. Model results suggest no net trend in the storage of water over land due to climate-driven changes but there are large interannual and decadal fluctuations. However, for the recent period 1993 to 2003, the small discrepancy between observed sea level rise and the sum of known contributions might be due to unquantified human-induced processes (e.g., groundwater extraction, impoundment in reservoirs, wetland drainage and deforestation).
Global sea level is projected to rise during the 21st century at a greater rate than during 1961 to 2003. Under the IPCC Special Report on Emission Scenarios (SRES) A1B scenario by the mid-2090s, for instance, global sea level reaches 0.22 to 0.44 meters above 1990 levels, and is rising at about 4 mm yr–1. As in the past, sea level change in the future will not be geographically uniform, with regional sea level change varying within about ±0.15 m of the mean in a typical model projection. Thermal expansion is projected to contribute more than half of the average rise, but land ice will lose mass increasingly rapidly as the century progresses. An important uncertainty relates to whether discharge of ice from the ice sheets will continue to increase as a consequence of accelerated ice flow, as has been observed in recent years. This would add to the amount of sea level rise, but quantitative projections of how much it would add cannot be made with confidence, owing to limited understanding of the relevant processes.
Will ice sheets melt with climate change?
The two major ice sheets are on Greenland and in the Antarctic. The Greenland Ice Sheet contains enough water to contribute about 7 meters to sea level, and the West Antarctic ice sheet (WAIS), which is the part of the Antarctic ice sheet most vulnerable to climate change, contains about 6 meters.
A sustained rise in local temperatures of about 3 °C, equivalent to a global-mean warming of about 1.5 °C, which is likely to be reached by the end of the century if man-made emissions are not controlled, would melt the Greenland Ice Sheet, although it is estimated that this would take a few thousand years. A major collapse of the WAIS is thought to be very unlikely during the 21st century, although recent measurements suggest that contributions to sea level rise from this source may be greater than previously estimated.
Are El Niños related to Global Warming?
El Niños are not caused by global warming. Clear evidence exists from a variety of sources (including archaeological studies) that El Niños have been present for thousands, and some indicators suggest maybe millions, of years. However, it has been hypothesized that warmer global sea surface temperatures can enhance the El Niño phenomenon, and it is also true that El Niños have been more frequent and intense in recent decades. Whether El Niño occurrence changes with climate change is a major research question.
What are the shortcomings of climate models?
Global climate models—the software packages that simulate the past, present, and future of our atmosphere—have grown in complexity and quality over the last 10 to 20 years. Yet even the earliest models of the 1960s, which were quite crude by today’s standards, showed that a doubling of carbon dioxide in the atmosphere could increase global temperature by around 5°F (3°C). That projection remains close to the modern consensus, and temperatures over the last 30 years have risen at a rate consistent with this early estimate.
The best climate models encapsulate the current understanding of the physical processes involved in the climate system, the interactions, and the performance of the system as a whole. They have been extensively tested and evaluated using observations. They are exceedingly useful tools for carrying out numerical climate experiments, but they are not perfect, and some models are better than others. Uncertainties arise from shortcomings in our understanding of climate processes operating in the atmosphere, ocean, land and cryosphere, and how to best represent those processes in models. Yet, in spite of these uncertainties, today's best climate models are now able to reproduce the climate of the past century, and simulations of the evolution of global surface temperature over the past millennium are consistent with paleoclimate reconstructions. This gives increased confidence in future projections. The shortcomings in our understanding of the processes involved in climate and how they are depicted in models arise from inadequate observations and theoretical underpinnings associated with the incredible complexity of dealing with scales from molecules and cloud droplets to the planetary-scale atmospheric circulation. These issues are addressed in several steps:
- Individual climate processes are dealt with as best as is possible given the understanding and computational limitations.
- The processes are assembled in models and then the model components are tested with strong constraints. The components include modules of the atmosphere, the oceans, the land and sea ice, and the land surface. These modules are coupled together to mimic the real world.
- The climate system model as a whole is then integrated in an unconstrained mode and thoroughly tested against observations.
One strong test is to simulate the annual cycle of seasonal variations (the changes in climate from winter to summer). Another is to simulate observed variability from one year to the next. Yet another is to simulate past climate, even going back in time thousands or millions of years tested against records from ice cores, tree rings, and other "proxy" data.
As our knowledge of the different components of the climate system and their interactions increases, so does the complexity of today's climate models. Also, many of the most pressing scientific questions regarding the climate system and its response to natural and anthropogenic forcings cannot be readily addressed with traditional models of the physical climate. One of the open issues for near-term climate change, for example, is the response of terrestrial ecosystems to increased concentrations of carbon dioxide. Will plants begin releasing carbon dioxide to the atmosphere in a warmer climate, thereby acting as a positive feedback, or will vegetation absorb more carbon dioxide and hence decelerate global warming? Related issues include the interactions among land use change, deforestation by biomass burning, emission of greenhouse gases and aerosols, weathering of rocks, carbon in soils, and marine biogeochemistry.
Exploration of these questions requires a comprehensive treatment of the integrative Earth system. In order to address these emerging issues, physical models are being extended to include the interactions of climate with biogeochemistry, atmospheric chemistry, ecosystems, glaciers and ice sheets, and anthropogenic environmental change. These new "Earth System Models", however, will require large investments in computing infrastructure before they can be fully utilized.
Don't all the government reviewers and negotiations over wording make the IPCC report a political, not a scientific, document?
The unique structure of the IPCC includes both scientific and governmental review, but the input of diplomats to the final report is designed to be distinct and different from that of scientists.
Scientists who are experts in their subject matter prepare the chapters that go into the full assessment reports. Those chapters are scrutinized by individual scientists and scientist panels, whose questions must be addressed before a chapter can be approved for inclusion. The chapters making up the 2007 draft report from IPCC Working Group I report run to over 1,600 pages.
It is only the Summary for Policymakers, typically around 30 pages, that receives a word-by-word review, during the final plenary session, by diplomats from almost every nation in the world. The lead authors of the report are on hand at the plenary to make sure that any changes are scientifically valid. The diplomats have a say in how the Summary for Policymakers is worded, but the scientists have the last word on what is said.
The result of the IPCC process is a report that carries the weight of formal approval by the world's governments as well as the authority of hundreds of participating scientists.
Weather forecasts aren't accurate for more than a few days ahead, so how can we possibly predict climate over the next 100 years?
Although they are made by the same sort of mathematical model, weather forecasts and climate predictions are really quite different. A weather forecast tells us what the weather (for example, temperature or rainfall) is going to be at a certain place and time over the next few days; it might say, for example, that there will be a band of heavy rain moving across Tokyo tomorrow mid-morning.
A climate prediction tells us about changes in the average climate, its variability and extremes. For example, it might say that the average temperature of summers in Tokyo in 40–60 years time will be 4 degrees higher than it is currently, it will enjoy on average 25% more rain in winter with three times the current number of heavy rainfall events, and 50% less rain in summer. It will not make a specific forecast such as: it will be raining in Tokyo on the morning of 15 October 2044.
Are climate change and ozone depletion part of the same thing?
Not really, although there are links between the two. The depletion of ozone in the stratosphere over Antarctica (the 'ozone hole') was first discovered by scientists from the British Antarctic Survey in the mid-1980s. It is caused mainly by emissions of man-made chlorofluorocarbons (CFCs), which find their way into the stratosphere where they decompose into chlorine compounds which destroy ozone each autumn. Despite the fact that emissions of CFCs have been very severely cut back by the Montreal Protocol, because they have a lifetime of order 100 years, their concentration in the atmosphere has only recently started to turn down, and the ozone hole is expected to remain as large as it is now for decades to come, before it slowly recovers.
Links with climate change are threefold. Firstly, the CFCs which deplete ozone, and also some of their ozone-friendly replacements, are greenhouse gases and so also contribute directly to global warming. Secondly, the reduction in stratospheric ozone, both over Antarctica and more generally globally, acts to cool climate slightly. Lastly, there is concern that increasing concentrations of CO2 from fossil-fuel burning, because it is cooling the stratosphere, aids the formation of the small particles in the stratosphere on which chemical reactions take place, and may be prolonging the ozone hole.
Earth’s surface, the lowest 8 km of the atmosphere and the upper 300m of ocean. Snow cover and ice extent have decreased, global-average sea-level has risen, the frequency of extremely high temperatures has increased and the frequency of extremely low temperatures has decreased.
Is there inconsistency between satellite records of warming and surface measurements?
Although there is solid evidence for global warming in the 20th century, much attention has focussed on the period since 1979 when satellite records became available. These records provide a global measure of temperature in the lower atmosphere. When this is compared with surface temperature measurements, the lower atmosphere appears to have warmed less than the Earth’s surface. This apparent inconsistency could be affected by three factors. Firstly, the lower atmosphere and the surface are affected differently by factors such as stratospheric ozone depletion, atmospheric aerosols and El Niño. Secondly, one might expect differences due to such a short sampling period. Thirdly, satellite measurements require a number of adjustments before they can be converted to temperature data. When corrections are made for some of these considerations, the satellite-based warming of the lower atmosphere since 1979 is 0.18°C per decade, which is almost exactly the same as the surface warming (0.17°C per decade).
- Intergovernmental Panel on Climate Change
- Earth and Sun Systems Laboratory (ESSL), National Center for Atmospheric Research (NCAR)
- CO2 Program, Scripps Institution of Oceanography
- National Climatic Data Center, National Oceanic and Atmospheric Administration
- University Corporation for Atmospheric Research
- Met Office
- World Climate Research Programme,World Meteorological Organization
- Department of Climate Change, Australian Government