Climate sensitivity: Analysis of feedback mechanisms

Abstract

We study climate sensitivity and feedback processes in three independent ways: (1) by using a three dimensional (3-D) global climate model for experiments in which solar irradiance S0 is increased 2 percent or CO2 is doubled, (2) by using the CLIMAP climate boundary conditions to analyze the contributions of different physical processes to the cooling of the last ice age (18K years ago), and (3) by using estimated changes in global temperature and the abundance of atmospheric greenhouse gases to deduce an empirical climate sensitivity for the period 1850–1980. Our 3-D global climate model yields a warming of ∼4°C for either a 2 percent increase of S0 or doubled CO2. This indicates a net feedback factor of f = 3–4, because either of these forcings would cause the earth's surface temperature to warm 1.2–1.3°C to restore radiative balance with space, if other factors remained unchanged. Principal positive feedback processes in the model are changes in atmospheric water vapor, clouds and snow/ice cover. Feedback factors calculated for these processes, with atmospheric dynamical feedbacks implicitly incorporated, are respectively fwater vapor ∼ 1.6, fclouds ∼ 1.3 and fsnow/ice ∼ 1.1 with the latter mainly caused by sea ice changes. A number of potential feedbacks, such as land ice cover, vegetation cover and ocean heat transport were held fixed in these experiments. We calculate land ice, sea ice and vegetation feedbacks for the 18K climate to be fland ice ∼ 1.2–1.3, fsea ice ∼ 1.2 and fvegetation ∼ 1.05–1.1 from their effect on the radiation budget at the top of the atmosphere. This sea ice feedback at 18K is consistent with the smaller fsnow/ice ∼ 1.1 in the S0 and CO2 experiments, which applied to a warmer earth with less sea ice. We also obtain an empirical estimate of f = 2–4 for the fast feedback processes (water vapor, clouds, sea ice) operating on 10–100 year time scales by comparing the cooling due to slow or specified changes (land ice, C02, vegetation) to the total cooling at 18K. The temperature increase believed to have occurred in the past 130 years (approximately 0.5°C) is also found to imply a climate sensitivity of 2.5–5°C for doubled C02 (f = 2–4), if (1) the temperature increase is due to the added greenhouse gases, (2) the 1850 CO2 abundance was 270±10 ppm, and (3) the heat perturbation is mixed like a passive tracer in the ocean with vertical mixing coefficient k ∼ 1 cm2 s−1. These analyses indicate that f is substantially greater than unity on all time scales. Our best estimate for the current climate due to processes operating on the 10–100 year time scale is f = 2–4, corresponding to a climate sensitivity of 2.5–5°C for doubled CO2. The physical process contributing the greatest uncertainty to f on this time scale appears to be the cloud feedback. We show that the ocean's thermal relaxation time depends strongly on f. The e-folding time constant for response of the isolated ocean mixed layer is about 15 years, for the estimated value of f. This time is sufficiently long to allow substantial heat exchange between the mixed layer and deeper layers. For f = 3–4 the response time of the surface temperature to a heating perturbation is of order 100 years, if the perturbation is sufficiently small that it does not alter the rate of heat exchange with the deeper ocean. The climate sensitivity we have inferred is larger than that stated in the Carbon Dioxide Assessment Committee report (CDAC, 1983). Their result is based on the empirical temperature increase in the past 130 years, but their analysis did not account for the dependence of the ocean response time on climate sensitivity. Their choice of a fixed 15 year response time biased their result to low sensitivities. We infer that, because of recent increases in atmospheric CO2 and trace gases, there is a large, rapidly growing gap between current climate and the equilibrium climate for current atmospheric composition. Based on the climate sensitivity we have estimated, the amount of greenhouse gases presently in the atmosphere will cause an eventual global mean warming of about 1°C, making the global temperature at least comparable to that of the Altithermal, the warmest period in the past 100,000 years. Projection of future climate trends on the 10–100 year time scale depends crucially upon improved understanding of ocean dynamics, particularly upon how ocean mixing will respond to climate change at the ocean surface.