Response time and climate change
Response time in the context of climate change refers to the duration it takes for natural systems to react to changes in forcing mechanisms, particularly radiative forcing. Radiative forcing, which is the net energy at the boundary between the Earth's surface and atmosphere, is influenced by both natural processes, like variations in Earth's orbit and solar radiation, and human activities that add greenhouse gases to the atmosphere. Different components of the Earth system, such as the troposphere and ocean, exhibit varying response times largely due to their heat capacities, with the ocean's response time being significantly slower than that of the atmosphere.
Understanding these response times is essential for predicting future climate changes. For instance, even if CO2 emissions were stabilized at 2000 levels, models indicate a continued rise in global temperatures due to the ocean's slow response. This phenomenon highlights the urgency of addressing climate change, as regional temperature variations depend on the specific response times of different environmental reservoirs. Additionally, geoengineering efforts aimed at mitigating climate change are being explored, including methods to reduce atmospheric CO2 or block solar radiation. However, these proposals raise concerns regarding their potential consequences, necessitating further research to assess their impacts and effectiveness. As climate scientists seek solutions, the complex interplay of response times and human intervention remains a crucial area of study in addressing climate change.
Subject Terms
Response time and climate change
Definition
The response time of a natural system is the amount of time it takes for the system to respond to a change in the forcing mechanisms. In the case of the climate system, the forcing mechanism of most importance is radiative forcing, which is defined as the net irradiance at the top of the (boundary between the and the stratosphere). There are many processes that can affect radiative forcing, including natural phenomena such as variations in the Earth’s orbit or in the output of solar radiation, or (human-induced) effects such as the addition of greenhouse gases to the atmosphere. Different components of the Earth system, such as the troposphere, stratosphere, or ocean, have different response times. These differences are largely controlled by heat capacity, which is the amount of heat required to change the temperature of a specific mass of the substance by 1° Celsius.
Heat capacity acts like a shock absorber, slowing the response rate of a system to a temperature change. The heat capacity of the ocean is approximately four times greater than that of the atmosphere, so that the response time of the ocean is much slower than that of the atmosphere. Response times of the atmosphere are typically on the scale of days to weeks, while that of the deepest parts of the ocean can exceed one thousand years. Variations in response time are also observed within each reservoir. For example, because the troposphere is coupled to the ocean—which acts as a brake for changes in the troposphere—its response time is typically on the order of weeks to a few months. In contrast, the response time of the is shorter, ranging from a few days to weeks.
Significance for Climate Change
The response time of different reservoirs is a critical aspect in attempts to predict future climate change. For example, the Intergovernmental Panel on Climate Change (IPCC) uses coupled ocean-atmosphere models to determine the effects of anthropogenic emissions on climate change. Even in a proposed in 2013 where CO2 emissions would be fixed at a level equal to that of the year 2000, the global temperature in 2100 was predicted to be 0.6° Celsius higher than the 2013 average. The temperature continued to rise due to the slow response time of the oceans. Regional variations in global temperature changes were also observed in models because the response time of the individual varied. The National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory showed the response time of the ocean varied with the direction of the change in radiative forcing. If the change was cooling (for example, decreased amounts of atmospheric CO2), the ocean responded approximately twice as fast as if the change was in a warming direction.
Because of the slow response time of the oceans, global temperatures were predicted to rise even if CO2 emissions were instantly held at levels consistent with those at the turn of the century, which led many scientists and policymakers to call for research into solutions to global warming. Geoengineering refers to any attempt by humans to control global climate. A variety of possible mechanisms have been proposed, designed to achieve one of two outcomes: decrease the amount of incoming solar radiation or reduce the amount of CO2 in the atmosphere. Proposed projects include chemical approaches such as adding iron to the ocean to reduce atmospheric concentrations of CO2 (“iron fertilization”) and pumping sulfur dioxide into the stratosphere to cause a higher rate of cloud formation. The Biden administration began implementing a climate intervention research project in 2022, which included geoengineering and was controversial among scientists. Some even questioned its legality under international law, but the project continued to advance in an effort to control climate change.
Other proposals include the installation of a “sunshade” made from stacks of very thin silicon nitride ceramic in space to block incoming solar radiation. Most climate scientists and geoengineering advocates agree that not enough is known about the negative consequences of the proposed geoengineering projects. Therefore, these scientists advocate conducting research to determine the potential consequences, such as regional variations in drought cycles or a decrease in atmospheric ozone. Finally, projects aimed at reducing the amount of incoming solar radiation may eventually lead to a decrease in global temperature, but may not directly affect other effects of increased concentrations of atmospheric CO2 that will remain a concern, such as ocean acidification.
Bibliography
Hansen, James, et al. “Earth’s Energy Imbalance and Climate Response Time.” Columbia, 22 Dec. 2022, www.columbia.edu/~jeh1/mailings/2022/EarthEnergyImbalance.22December2022.pdf. Accessed 13 Dec. 2024.
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.
Kunzig, Robert. “A Sunshade for Planet Earth.” Scientific American 299, no. 5 (November, 2008): 46-55.
Milman, Oliver. “Can Geoengineering Fix the Climate? Hundreds of Scientists Say Not so Fast.” The Guardian, 25 Dec. 2022, www.theguardian.com/environment/2022/dec/25/can-controversial-geoengineering-fix-climate-crisis. Accessed 13 Dec. 2024.
Oldfield, Frank. Environmental Change: Key Issues and Alternative Perspectives. New York: Cambridge University Press, 2005.
Samset, B. H., et al. “Delayed Emergence of a Global Temperature Response after Emission Mitigation.” Nature Communications, vol. 11, 2020. doi.org/10.1038/s41467-020-17001-1. Accessed 13 Dec. 2024.
Schellnhuber, Hans, et al., eds. Avoiding Dangerous Climate Change. New York: Cambridge University Press, 2006. Stjern, Camilla W., et al. "The Time Scales of Climate Change Responses to Carbon Dioxide and Aerosols." AMS, 2 May 2023, journals.ametsoc.org/view/journals/clim/36/11/JCLI-D-22-0513.1.xml. Accessed 13 Dec. 2024.