Climate sensitivity
Climate sensitivity refers to the Earth's response to changes in greenhouse gas concentrations, particularly carbon dioxide (CO2). It is primarily quantified by the concept of equilibrium climate sensitivity (ECS), which measures the increase in global average surface temperature resulting from a doubling of atmospheric CO2 concentrations from preindustrial levels. This change in temperature arises from a combination of direct radiative forcing and various internal climate feedback mechanisms, such as changes in water vapor, albedo, and cloud cover. The ECS is estimated to likely fall between 2°C and 4.5°C, with a most probable value around 3°C. Understanding ECS is crucial for predicting long-term climate change impacts, as even small differences in its estimate can lead to significantly different climate outcomes.
Methods for estimating ECS include direct observations of past climate, expert assessments, and variations of climate models compared against historical data. However, the upper limit of ECS remains difficult to determine, and there are limitations to its applicability, such as its dependence on specific climate conditions and the timeframe of radiative forcing changes. For shorter-term projections, the transient climate response (TCR) is used to assess how the climate reacts to changes over hundreds of years, distinguishing it from the longer-term equilibrium perspective of ECS. Overall, ECS plays a vital role in understanding and forecasting climate change, although uncertainties persist that complicate projections.
Climate sensitivity
Definition
Global climate is a complex system that reacts to changes in its components, such as atmospheric carbon dioxide (CO2) concentration. A change in the atmospheric concentration of CO2 may cause a change in the radiation balance of the Earth; such a change is called “radiative forcing.” Many changes can cause radiative forcing, including changes in greenhouse gas (GHG) concentration, the output of the Sun, ice cover, and aerosol concentration. In response to a change in the Earth’s radiation balance, the planet’s temperature will change until global energy balance is restored. How much the global temperature changes depends on internal feedbacks in the Earth’s climate system that cause net amplification of the initial radiative forcing. Internal climate feedbacks include changes in water vapor, lapse rate, albedo, and clouds.
Equilibrium climate sensitivity (ECS) is a useful summary statistic of the behavior of the Earth’s climate system. ECS is defined as the change in equilibrium of global average surface temperature in response to a doubling of the atmospheric concentration of CO2 from preindustrial levels (from 280 parts per million to 560 parts per million). A doubling of atmospheric CO2 causes radiative forcing of about 3.7 watts per square meter by increasing long-wave radiative absorption. ECS is not a simple measure of the amount of thermal energy added to Earth’s climate system by the CO2 alone, because climate changes cause feedback loops. For example, melting ice decreases Earth’s albedo, causing the planet’s surface to absorb more heat and to further increase thermal energy. If there were no internal climate feedbacks, then ECS would be about 1.2° Celsius. However, because of internal climate feedbacks, ECS is likely between 2° Celsius and 4.5° Celsius.
ECS allows scientists to assess the change in average global temperature after the Earth has reached equilibrium over several thousand years. However, it is computationally intensive to run complex global climate models to equilibrium. Thus, a modified concept,effective climate sensitivity, has been developed as an approximation of ECS. Effective climate sensitivity is calculated by estimating the climate feedback parameter at a specific point in time during transient climate conditions (not at equilibrium) using estimates of ocean heat storage, radiative forcing, and surface temperature change. Some studies find that effective climate sensitivity calculations underestimate the true ECS of a given model.
Significance for Climate Change
ECS is used to summarize and compare different climate models, as well as to combine information from models, historical records, and paleoclimate reconstructions. It is immensely important for making, understanding, and reacting to projections of future climate change. Every 1° Celsius difference in ECS can imply vastly different impacts on the planet over the long term.
A large number of studies have estimated ECS using a variety of methods, models, and data. There are four main categories of strategies used to estimate ECS. One method is to estimate ECS directly from observations of past climate changes. Another is to compile expert opinions. A third is to take a single climate model, create multiple versions by varying its parameters, and then compare the climate simulated by each version with climate observations to determine which is most likely. A final strategy is to combine the results of multiple methods into a single probability distribution. There are two main sources of climate observations used in this type of research: modern instrumental observations (after 1850), and paleoclimate reconstructions over the past thousands or millions of years.
After considering all the available research, the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) concluded that ECS is likely (with a probability of greater than 66 percent) to be between 2° Celsius and 4.5° Celsius, very likely (greater than 90 percent probability) to be larger than 1.5° Celsius, and most likely to have a value of about 3° Celsius. The estimated range of ECS has been relatively stable over a thirty-year period: A range of 1.5° Celsius to 4.5° Celsius was proposed in 1979 in a report by the National Academy of Sciences. Scientists have improved the certainty of their estimates of the lower limit of climate sensitivity and of the transient climate response. The upper limit of ECS, however, remains difficult to quantify as a result of nonlinearities that cause a skewed probability distribution. The persistence of such large uncertainty in the value of ECS is a significant barrier to narrowing the range of projections of future climate change.
There are several important limitations to the concept of ECS. It is potentially dependent upon the state of the climate system and upon the rate and magnitude of radiative forcing: A doubling of CO2 versus a halving of CO2 may not cause the same magnitude of temperature change. Additionally, different forcing mechanisms can have different sensitivities to radiative forcing, so ECS values may be specific to changes in CO2. Lastly, ECS quantifies equilibrium temperature change over thousands of years, so it does not give direct projections for future climate changes over periods of hundreds of years. A separate summary statistic, transient climate response (TCR), was developed to compare the transient responses of climate models and provide shorter-term projections.
Bibliography
Edwards, T. L., et al. “Using the Past to Constrain the Future: How the Palaeorecord Can Improve Estimates of Global Warming.” Progress in Physical Geography 31, no. 5 (October, 2007): 481-500. Provides an accessible overview of ECS research, with useful tables categorizing studies and a focus on the role of paleoclimate reconstructions in ECS estimations.
Intergovernmental Panel on Climate Change. Climate Change, 2007—The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Edited by Susan Solomon et al. New York: Cambridge University Press, 2007. The IPCC provides an extensive overview of the scientific literature on ECS. Chapters 6, 8, 9, and 10 are most relevant.
Knutti, R., and G. C. Hegerl. “The Equilibrium Sensitivity of the Earth’s Temperature to Radiation Changes.” Nature Geoscience 1, no. 11 (November, 2008): 735-743. Provides an updated technical overview of the scientific literature on ECS, with useful figures comparing different estimates.