Feedback temperature dependence determines the risk of high warming

Abstract

The long‐term warming from an anthropogenic increase in atmospheric CO2 is often assumed to be proportional to the forcing associated with that increase. This paper examines this linear approximation using a zero‐dimensional energy balance model with a temperature‐dependent feedback, with parameter values drawn from physical arguments and general circulation models. For a positive feedback temperature dependence, warming increases Earth's sensitivity, while greater sensitivity makes Earth warm more. These effects can feed on each other, greatly amplifying warming. As a result, for reasonable values of feedback temperature dependence and preindustrial feedback, Earth can jump to a warmer state under only one or two CO2 doublings. The linear approximation breaks down in the long tail of high climate sensitivity commonly seen in observational studies. Understanding feedback temperature dependence is therefore essential for inferring the risk of high warming from modern observations. Studies that assume linearity likely underestimate the risk of high warming.

Climate Sensitivity on Geological Timescales Controlled by Nonlinear Feedbacks and Ocean Circulation

“…We note that λ =−1 W·m −2 ·K −1 corresponds to a linear climate sensitivity of 3.7 °C per CO 2 doubling, which is within the range estimated by the IPCC. We also note that a =0.04 W·m −2 ·K −2 is within the range estimated by Bloch‐Johnson et al () of −0.04 ≤ a ≤ 0.06 W·m −2 ·K −2 from various climate model simulations carried out at multiple CO 2 concentrations. Finally, we note that a value of e geog close to unity also supports the assumption that surface albedo forcing associated with ocean area is a mechanism for variations in F geog that is consistent with simple radiative forcing considerations.…”

“…This is clearly at odds with the variable climate sensitivity in the full climate model (Figure c), as also highlighted by the non‐unity slope of the best fit line between the ×2 and ×4 simulations (supporting information Figure S4). As such, we introduce a nonlinear parameter, a (W·m −2 ·K −2 ), into equation , following Bloch‐Johnson et al (), such that …”

Deep time evidence for climate sensitivity increase with warming

“…Our findings support the notion that as p CO 2 and global temperatures continue to rise, there may be additional warming as CS may increase despite less ice‐albedo feedback as snow and sea ice cover diminish. If warming increases CS, greater CS makes the Earth warm more, amplifying the warming [ Bloch‐Johnson et al ., ]. Thus, our results further underline the need to limit ongoing global warming by greatly reducing anthropogenic greenhouse gas emissions as soon as possible.…”

Accounting for Changing Temperature Patterns Increases Historical Estimates of Climate Sensitivity

“…Determining whether values toward the extremes of these bounds are plausible would require further understanding of the pattern effect or assessing and combining other lines of evidence, such as from process understanding (see Stevens et al, 2016). This is important because a higher EffCS increases the risk of state-dependent feedbacks and large warmings (Bloch-Johnson et al, 2015).…”