Fast atmosphere‐ocean model runs with large changes in CO₂

Abstract

How does climate sensitivity vary with the magnitude of climate forcing? This question was investigated with the use of a modified coupled atmosphere‐ocean model, whose stability was improved so that the model would accommodate large radiative forcings yet be fast enough to reach rapid equilibrium. Experiments were performed in which atmospheric CO2 was multiplied by powers of 2, from 1/64 to 256 times the 1950 value. From 8 to 32 times, the 1950 CO2, climate sensitivity for doubling CO2 reaches 8°C due to increases in water vapor absorption and cloud top height and to reductions in low level cloud cover. As CO2 amount increases further, sensitivity drops as cloud cover and planetary albedo stabilize. No water vapor‐induced runaway greenhouse caused by increased CO2 was found for the range of CO2 examined. With CO2 at or below 1/8 of the 1950 value, runaway sea ice does occur as the planet cascades to a snowball Earth climate with fully ice covered oceans and global mean surface temperatures near −30°C.

Radiative forcing at high concentrations of well‐mixed greenhouse gases

“…Under such large climate changes, substantial nonsmooth climate feedbacks are expected, and the proportionality between radiative forcing and mean surface temperature will weaken. Such climates are an active area of research with general circulation models [e.g., Abbot et al, 2013;Russell et al, 2013]. Nonetheless, radiative forcings and a climate sensitivity parameter of ≈0.5 K∕(W m −2 ) [IPCC, 2001] provide a good first-approximation estimate of climate change and are the best way of comparing the relative efficacy of different greenhouse gases.…”

A general framework for understanding the response of the water cycle to global warming over land and ocean

“…That approach clearly revealed the underlying basis of the projected warming that occurs in the climate models. In particular a relatively small top of the atmosphere forcing due to CO 2 and other long-lived greenhouse gases is amplified, mostly by water vapour feedback, into a large increase in the incoming long-wave irradiance at the surface (Held and Soden, 2000;Russell et al, 2013). Paradoxically, there is not yet enough warming to be able to confidently test the projected changes against global observations of P and atmospheric water vapour (Liepert and Previdi, 2009;Vonder Haar et al, 2012).…”

“…Hence for a warming of 2.8 K, the projected change in the mass of water in the atmosphere is (30 × 0.07 × 2.8 =) 5.9 mm (equivalent depth of liquid water). Taken over the 100 yr period under consideration here, the change is too small (= 5.9 mm/100 a = 0.059 mm a −1 ) to have a measurable impact on either the global mean annual P or E. This raises an interesting point -the absolute change in water content of the atmosphere plays little role in the global mass balance but that same change leads to a substantial fraction of the global warming projected by the climate models via the so-called positive water vapour feedback (Held and Soden, 2000;Russell et al, 2013). We will return to this important point in the Discussion and Conclusions (Sect.…”

The role of long-lived greenhouse gases as principal LW control knob that governs the global surface temperature for past and future climate change

“…The heavy red arrow angling towards figure top depicts a possible 'runaway' danger zone where positive CO 2 feedbacks from existing CO 2 reservoirs have the potential to exceed human capacity to maintain control over global climate change. Model results were calculated using the Russell et al (2013) 27-layer, 48)38 coupled fast atmosphere-ocean model (FAOM). Based on attribution analysis, the feedback contribution to the greenhouse strength (yellow) is subdivided into its water vapour and cloud components.…”