Understanding the climate systems used in climate models

Forcing the issue

If world energy consumption continues along projected paths, the carbon dioxide concentration levels could more than double by 2050. The impact this would have on world’s climate is the ∞-dollar question and focus of an army of scientists who devise and study climate models. (See slide 12 of the Power Point Presentation, “Global Energy Perspective,” by Nathan Lewis.)

Broadly defined, climate models are numerical representations of the climate system based on the physical, chemical and biological properties of its components, their interactions and feedback processes. Climate models are derived from fundamental physical laws, such as Newton’s laws of motion, which are then subjected to physical approximations appropriate for the large-scale climate system. (See Climate Change 2007: The Physical Science Basis, published by the Intergovernmental Panel on Climate Change, Chapter 8, “Climate Models and Their Evaluation,” pages 596, 943.)

“A climate model is a very complex system, with many components. The model must of course be tested at the system level, that is, by running the full model and comparing the results with observations. Such tests can reveal problems, but their source is often hidden by the model’s complexity,” according to the IPCC report (pg. 594).

The most important use of climate models in the context of global climate change is their ability to predict temperature changes when certain conditions – such as an increase in carbon dioxide levels – are “forced” upon the system. This is known as climate sensitivity.

The current generation of general circulation climate models calculate that a doubling of the carbon dioxide levels in the atmosphere would cause the earth’s temperature to rise between 2.1 and 4.4 degrees Celsius, with the mean value of 3.2 degrees. In technical terms, these figures represent the equilibrium climate sensitivity – the global annual mean surface temperature change experienced by the climate system after it has attained a new equilibrium in response to a doubling of atmospheric carbon dioxide concentrations. (Climate Change 2007, pgs. 629-631)

The climate system

The climate system is a highly complex set of systems consisting of five major components: the atmosphere, the hydrosphere (oceans, rivers, lakes), the cryosphere (the arctic and Antarctic), the land surface and the biosphere (all ecosystems and living organisms on the land and oceans). The climate system evolves over time due to its own internal dynamics and due to external forcings such as volcanic eruptions, solar variations and anthropogenic forcings by humans. (pg. 943)

“The climate system includes a variety of physical processes, such as cloud processes, radiative process and boundary-layer processes, which interact with each other on many temporal and spatial scales,” according to the IPCC report (pg. 602). “Due to the limited resolutions of the models, many of these processes are not resolved adequately by the model grid and must therefore be parameterized.”

In climate models, parameterization refers to the technique of representing processes that cannot be explicitly resolved at the spatial or temporal resolution of the model. The differences between paramaterizations are an important reason why climate model results differ.

“For models to simulate accurately the global distribution of the annual and diurnal cycles of surface temperature, they must, in the absence of compensating errors, correctly represent a variety of processes,” according to the report (pg. 608). “The large-scale distribution of annual mean surface temperature is largely determined by the distribution of insulation, which is moderated by clouds, other surface heat fluxes and transport of energy by the atmosphere and to a lesser extent by the ocean.”

The atmosphere and clouds

The atmosphere is the gaseous envelope surrounding the earth. The dry atmosphere is about 78 percent nitrogen and 20 percent oxygen, with additional trace gases, greenhouse gasses and aerosols. Greenhouse gasses in the atmosphere include .035 percent carbon dioxide and 1 percent water vapor. (All figures are measured in volume mixing ratios – the ratio of the number of moles of a constituent in a given volume to the total number of moles of all constituents in that volume.)

Cloud processes affect the climate system by regulating the flow of radiation at the top of the atmosphere, by producing precipitation, by accomplishing rapid and sometimes deep redistributions of atmospheric mass and through additional mechanisms. Cloud feedbacks are climate feedbacks involving changes in any of the properties of clouds as a response to other atmospheric changes. Understanding cloud feedbacks and determining their magnitude requires an understanding of how a change in climate may affect the spectrum of cloud types, the cloud fraction and height and the radiative properties of clouds, and an estimate of the impact of these changes on the earth’s radiation. (pgs. 602, 944)

Clouds, water vapor, and temperature all interact strongly with one another in the earth’s climate system. The processes of these systems are considered key elements in climate sensitivity. (pg. 633)

“At present, cloud feedbacks remain the largest source of uncertainty in climate sensitivity estimates,” according to the IPCC report (pg. 944).

A cloudy picture

The differing climate sensitivity readings – from 2.1 to 4.4 degrees among the different Atmosphere-Ocean General Circulation models – are the focus of considerable scientific research and analysis. According the IPCC report, cloud radiative feedbacks are the most culpable for this spread.

“The relatively poor simulation of these clouds in the present climate is a reason for some concern,” according to the report (pg. 593). “The response to global warming of deep convective clouds is also a substantial source of uncertainty in projections since current models predict different responses of these clouds. It is not yet possible to determine which estimates of the climate change cloud feedbacks are the most reliable.”

Clouds exert two competing effects on the sun’s radiation to the earth: Clouds reflect solar radiation back into space – a process known as albedo – blocking the sun’s energy from reaching the earth; but clouds also trap infrared radiation emitted by the earth’s surface and the lower troposphere, thus having a warming effect (the greenhouse effect of clouds). The balance between these two forces depends on many factors. In current conditions on the earth, clouds exert a cooling effect on the climate. In technical terms, the “cloud radiative forcing” is said to be negative.

In response to global warming, the cooling effect of clouds on the climate might be weakened OR enhanced. In experiments where double the amount atmospheric carbon dioxide is forced into the system, climate models exhibit a broad range of global cloud feedbacks, with about half of the models predicting a negative cloud radiative forcing in response to global warming, and half predicting the opposite. (pgs. 635-637)

“In many climate models, details in the representation of clouds can substantially affect the model estimates of cloud feedback and climate sensitivity. Moreover, the spread of climate sensitivity estimates among current models arises primarily from inter-model differences in cloud feedbacks. Therefore, cloud feedbacks remain the largest source of uncertainty in climate sensitivity estimates,” according to the report. (pg. 636)

“Despite some advances in the understanding of the physical processes that control the cloud response to climate change and in the evaluation of some components of cloud feedbacks in current models, it is not yet possible to assess which of the model estimates of cloud feedback is the most reliable,” the report concludes (pg. 638). “Although the errors in the simulation of the different cloud types may eventually compensate and lead to a prediction of the mean cloud radiative forcing in agreement with observations, they cast doubts on the reliability of the model cloud feedbacks.

Air, water and convection

At the heart of understanding what determines the regional distribution of precipitation over land and oceans in the tropics is atmospheric convection and its interaction with large-scale circulation, according to the IPCC report. (pg 612)

The earth’s climate system is driven by solar radiation that heats the earth’s surface, and this heat is then transferred to the atmosphere by processes that are mostly convective. In the earth’s atmosphere and oceans, convection is vertical motion of air or water driven by buoyancy, usually caused by near-surface cooling or increases in salinity in the case of the ocean and near-surface warming in the atmosphere.

When a parcel of air is heated, it expands, becoming less dense and is pushed upward by buoyancy, carrying the heat energy upward with it. The air then cools, so it contracts, and sinks. The cycle then repeats with the cold air reheating and rising again. Since it cannot sink through the rising air beneath it, it moves laterally and then begins to sink. These convection currents cause local breezes, winds, thermals, cyclones and thunderstorms, and at a larger scale, produce the global atmospheric circulation features.

Water vapor feedback is the most important feedback enhancing climate sensitivity. (pg. 593)

Terrestrial processes

This includes processes on the surface of the earth. A key role of the land surface is the storage of soil moisture and the control its evaporation. Recent studies suggest that summer precipitation strongly depends on surface processes, especially in the simulation of regional extremes. Soil moisture anomalies have been correlated with the African monsoon and the 2003 European heat wave. (pgs. 604-5)

Surface temperature is strongly coupled with the atmosphere above it. This is especially evident at mid-latitudes, where migrating cold fronts and warm fronts can cause relatively large swings in surface temperature. (pg. 609)

The 23 Atmosphere-Ocean General Circulation include few multi-modal analyses of terrestrial processes. The Multi-Modal Data set has allowed hundreds of researchers from outside the modeling groups to scrutinize the models from a variety of perspectives. (Climate Change 2007, pgs. 594, 604. Also see the Lawrence Livermore National Laboratory’s Program for Climate Model Diagnosis and Inter-comparison.)

“Evaluation of the land surface component in coupled models is severely limited by the lack of suitable observations,” according to the IPCC report (pg. 617). “The terrestrial surface plays key climatic roles in influencing the partitioning of available energy between sensible and latent heat fluxes, determining whether water drains or remains available for evaporation, determining the surface albedo and whether snow melts or remains frozen, and influencing surface fluxes of carbon and momentum. Few of these can be evaluated at large spatial or long temporal scales.”

Oceans and oscillations

Oceans absorb and store vast amounts of the sun’s radiative heat, thus moderating the seasonal cycle of surface temperatures. Ocean currents transport heat from the tropics northward in a process called “meridional overturning circulation.” Waters warmed in the tropics are driven northward where the surface waters are cooled, causing the water to become denser and sink and then again flow southward at deeper levels. This process is responsible for the relatively mild climate of the North Atlantic and the British Isles. (pgs. 615, 949)

Oceanic heat fluxes have large seasonal variations which lead to large variations in the seasonal storage of heat by the oceans, especially in the mid-latitudes. The oceanic heat storage tends to damp and delay the seasonal cycle of surface temperature. Changes around continental margins are very important for regional climate change. (pgs. 614, 603)

The El Niño-Southern Oscillation refers to the warming of equatorial Pacific waters from the western coast of South America to the International Dateline. This oceanic event is associated with a fluctuation of a global-scale tropical and subtropical surface pressure pattern called the Southern Oscillation. During an El Niño-Southern Oscillation event, which occurs every two to seven years, the prevailing trade winds weaken, reducing upwelling and altering ocean currents, causing sea surface temperatures to warm and impacting the wind and precipitation patterns in the tropical Pacific and beyond. (pg. 945. Also see the National Oceanic and Atmospheric Administration El Niño page.)

“Systematic biases have been found in most climate models’ simulation of the Southern Ocean. Since the Southern Ocean is important for ocean heat uptake, this results in some uncertainty in transient climate response,” according to the IPCC report (pgs. 591, 592).

“In the tropics, there has been an overall improvement in the Atmosphere-Ocean General Circulation Models simulation of the spatial pattern and frequency of the El Niño-Southern Oscillation, but problems remain in simulating its seasonal phase locking and the asymmetry between El Niño and La Niña episodes. (Also see the National Oceanic and Atmospheric Administration La Niña page.)

The Madden-Julian Oscillation is an intraseasonal fluctuation or “wave” that propagates along the equator from the western Indian Ocean to the central Pacific. This phenomenon is responsible for the majority of weather variability in these regions.

Simulation of the Madden-Julian Oscillation remains unsatisfactory, according to the IPCC report (pgs. 591, 625).

The cryosphere

The cryosphere is the component of the climate system consisting of all snow, ice and frozen ground (including permafrost) on and beneath the surface of the earth and ocean. Currently, ice permanently covers 10 percent of the earth’s land surface, with only a tiny fraction occurring outside Antarctica and Greenland. Ice covers about 7 percent of the oceans in the annual mean.

An important property of snow and ice is its high surface albedo – the fraction of solar radiation reflected by a surface or object. “Because up to 90 percent of the incident solar radiation is reflected by snow and ice surfaces, while only about 10 percent is reflected by the open ocean or forested lands, changes in snow and ice cover are important feedback mechanisms in climate change,” according to the IPCC report (pg. 43).

The cryosphere stores about 75 percent of the world’s freshwater. Since the change from ice to liquid water occurs at specific temperatures, ice is a component of the climate system that could be subject to abrupt change following sufficient warming. During the 20th century, glaciers and ice caps have experienced widespread mass losses and have contributed to sea level rise. The maximum area covered by seasonally frozen ground decreased by about 7 percent in the northern hemisphere over the latter half of the 20th century. (pgs. 43-46)

In climate models, the magnitude of cryospheric feedbacks in climate models remains uncertain, contributing to the range of model climate responses at mid-to high latitudes. Understanding and evaluating sea ice feedbacks is complicated by the strong coupling to polar cloud processes and ocean heat and freshwater transport. Scarcity of observations in polar regions also hampers evaluation. (pg. 593)

Going polar

The earth’s polar regions, known as the cryosphere, play a significant role in climate sensitivity. Climate models respond to increases in atmospheric concentrations of greenhouse gasses with increased temperatures in the polar regions. Correspondingly, the increased temperatures cause a melting of snow and sea ice. (pg. 638)

Snow and ice-covered surfaces have a high albedo – that is to say, these highly reflective frozen surfaces in the polar regions reflect back most of the energy from the sun. But with the retreat of polar glaciers, more solar radiation is absorbed by the ground and water rather than reflected back. Therefore, global warming decreases the albedo feedback of the cryosphere, causing the earth’s overall albedo to decrease and more solar radiation to be absorbed – warming the earth still further. (pg. 941)

Ice sheet models are used in calculations of long-term warming and sea level scenarios. Sea ice components of current Atmospheric-Ocean General Circulation Models usually predict ice thickness (or volume), fractional cover, snow depth, surface and internal temperatures.

“The magnitude and spatial distribution of the high-latitude climate changes can be strongly affected by sea ice characteristics, but evaluation of sea ice in models is hampered by insufficient observations of some key variables. Even when sea ice errors can be quantified, it is difficult to isolate their causes, which might arise from deficiencies in the representation of the sea ice itself, but could also be due to flawed simulation of the atmospheric and oceanic fields at high latitudes that drive ice movement,” according to the IPCC report (pg. 616).

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