Climate Sensitivity And Decadal Temperature Variability

[bluewave]

Summary of key findings:

https://andthentheresphysics.wordpress.com/

There are some who argue that natural/internal variability can play a role in driving long-term warming, and – hence – could explain a substantial fraction of recent warming. This, however, creates a bit of a paradox; if the system responds strongly to internally-driven warming, then it should also respond strongly to externally-driven warming. Consequently, we’d expect climate sensitivity to be high which would then make it difficult for a large part of our recent warming to be due to natural/internal variability.

 

Credit: Figure 1b from Nijsse et al. (2019).

The reason I thought I’d mention this again is that I came across a recent paper by Femke Nijsse and colleagues that consider this issues. The paper is called Decadal global temperature variability increases strongly with climate sensitivityand the title pretty much gives away the punch-line. The paper shows that models that are more sensitive to GHGs emissions (that is, higher equilibrium climate sensitivity (ECS)) also have higher temperature variability on timescales of several years to several decades, which is illustrated in the figure on the right. 

 

The paper also points out that

high-sensitivity climates, as well as having a higher chance of rapid decadal warming, are also more likely to have had historical ‘hiatus’ periods than lower-sensitivity climates.

and consequently, that

the slowdown in global warming during the period 2002–2012 was more likely in a high-ECS world.

So, rather than the supposed slowdown being an indication of a low climate sensitivity, it could well imply the opposite. 

The paper then concludes that

[a]chieving a better consensus on the risk that we live in a high-ECS climate is therefore of critical importance to both the climate mitigation challenge and also to inform efforts to build resilience to climate variability.

Full paper:

https://www.nature.com/articles/s41558-019-0527-4.epdf?author_access_token=J-KyNPgaJjxqrOa7xDZqZdRgN0jAjWel9jnR3ZoTv0OBndQURhCsdUJ8UFlXX4uk6-ZhfEDlQ1ZVbSTbVVkwvTm8iW6Edp92vOc-lxlZOU0EYxumMyxUwh_xP8lBAiYB3-N-mPkH8GtRI4DbWmndkA%3D%3D

Abstract

Climate-related risks are dependent not only on the warming trend from GHGs, but also on the variability about the trend. However, assessment of the impacts of climate change tends to focus on the ultimate level of global warming1, only occasionally on the rate of global warming, and rarely on variability about the trend. Here we show that models that are more sensitive to GHGs emissions (that is, higher equilibrium climate sensitivity (ECS)) also have higher temperature variability on timescales of several years to several decades2. Counter-intuitively, high-sensitivity climates, as well as having a higher chance of rapid decadal warming, are also more likely to have had historical ‘hiatus’ periods than lower-sensitivity climates. Cooling or hiatus decades over the historical period, which have been relatively uncommon, are more than twice as likely in a high-ECS world (ECS = 4.5 K) compared with a low-ECS world (ECS = 1.5 K). As ECS also affects the background warming rate under future scenarios with unmitigated anthropogenic forcing, the probability of a hyper-warming decade—over ten times the mean rate of global warming for the twentieth century—is even more sensitive to ECS.