Graph showing how Earth’s climate sensitivity evolves to a doubling of atmospheric carbon dioxide over different timescales, starting at close to 2°C warming and then rising to about 3°C warming after a decade. Graphic: Goodwin, 2018 / Earth’s Future

By Dana Nuccitelli
24 September 2018

(The Guardian) – We’re currently on pace to double the carbon dioxide-equivalent (including other greenhouse gases) in the atmosphere by around mid-century.  Since the late 1800s scientists have been trying to answer the question, how much global warming will that cause?

In 1979, top climate scientists led by Jule Charney published a report estimating that if we double the amount of carbon dioxide in the atmosphere from pre-industrial levels of 280 ppm to 560 ppm, temperatures will warm by 3 ± 1.5°C.  Four decades later, ‘climate sensitivity’ estimates remain virtually unchanged, but some climate contrarians have argued that the number is at the low end of that range, around 2°C or less.

It’s an important question because if the contrarians are right, the 2°C resulting global warming would represent significantly less severe climate change consequences than if mainstream climate scientists are right and temperatures rise by 3°C.  It would also mean our remaining carbon budget for meeting the 2°C Paris target is about twice as large than if the mainstream consensus is right.  If the consensus is correct, we’re on pace to blow through the remaining Paris carbon budget by around 2030.

Another nail in the contrarian ‘low sensitivity’ coffin

Studies published in March 2014, May 2014, and December 2015 identified two critical flaws in the contrarians’ preferred so-called ‘energy balance model’ approach: it doesn’t account for the fact that Earth’s sensitivity can change over time, for example as large ice sheets continue to melt, or that the planet responds differently to different climate ”forcings”.

Last week, the journal Earth’s Future published a study by the University of Southampton’s Philip Goodwin that took both of these factors into account.  Goodwin ran climate model simulations treating every forcing separately, including changes in greenhouse gases, solar activity, particulates from volcanic eruptions, and from human fossil fuel combustion.  For each, he included feedbacks from changes in factors like atmospheric water vapor, clouds, snow, and sea ice, including how these factors change over different timescales, as Goodwin explained:

I ran 10 million simulations with a relatively simple climate model. These 10 million simulations each used different climate feedback strengths, and so the way that climate sensitivity responded over time was different in each simulation.  To check which of the 10 million simulations were most realistic, I checked each simulation against observations of warming in the atmosphere and ocean up to the present day. I kept only the simulations that agreed with the observations for the real world.

This left 4600 simulations, where the values of the climate sensitivity (and changes in climate sensitivity over different timescales) agree with the atmosphere and ocean warming observed so far. It is from these final 4600 simulations that I evaluate how the climate sensitivity evolves over time. […]

We are indeed on track to burn through the remaining Paris carbon budget by 2030, and under current international climate policies, we’re most likely headed for about 3.4°C warming by 2100. [more]

New study reconciles a dispute about how fast global warming will happen


ABSTRACT: The Earth's climate sensitivity to radiative forcing remains a key source of uncertainty in future warming projections. There is a growing realisation in recent literature that research must go beyond an equilibrium and CO2‐only viewpoint, towards considering how climate sensitivity will evolve over time in response to anthropogenic and natural radiative forcing from multiple sources. Here, the transient behaviour of climate sensitivity is explored using a modified energy balance model, in which multiple climate feedbacks evolve independently over time to multiple sources of radiative forcing, combined with constraints from observations and from the Climate Model Intercomparison Project phase 5 (CMIP5). First, a large initial ensemble of 107 simulations is generated, with a distribution of climate feedback strengths from sub‐annual to 102 year timescales constrained by the CMIP5 ensemble; including the Planck feedback, the combined water‐vapour lapse‐rate feedback, snow and sea‐ice albedo feedback, fast cloud feedbacks, and the cloud response to SST‐adjustment feedback. These 107 simulations are then tested against observational metrics representing decadal trends in warming, heat and carbon uptake, leaving only 4.6×103 history‐matched simulations consistent with both the CMIP5 ensemble and historical observations. The results reveal an annual‐timescale climate sensitivity of 2.1 °C (ranging from 1.6 to 2.8 °C at 95 percent uncertainty), rising to 2.9 °C (from 1.9 to 4.6 °C) on century timescales. These findings provide a link between lower estimates of climate sensitivity, based on the current transient state of the climate system, and higher estimates based on long‐term behaviour of complex models and palaeoclimate evidence.

Plain Language Summary

The Earth's climate sensitivity is a measure of how much the average surface temperature will increase if atmospheric carbon dioxide levels are doubled. There is currently a wide variation in estimates of the Earth's climate sensitivity at equilibrium, from low estimates around 1.5 {degree sign}C to high estimates around 4.5 {degree sign}C. Many different climate processes affect the value of the climate sensitivity, for example the responses to surface warming of clouds, atmospheric water vapor, and changes in the reflectivity of Earth's surface as snow and ice melt. These processes occur on different timescales, for example it takes days for water vapor to change in the atmosphere, but much longer to melt a large ice sheet. This study applies a range of observational constraints to climate model simulations in order to constrain the Earth's climate sensitivity, considering how the climate sensitivity varies on different timescales. A best estimate for climate sensitivity is found to be 2.1 °C (with uncertainty ranging from 1.6 to 2.8 °C) over yearly timescales. However, climate sensitivity increases to 2.9 °C (ranging from 1.9 to 4.6 °C) on century timescales, affecting future anthropogenic warming.

On the time evolution of climate sensitivity and future warming

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