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Ranges of equilibrium climate sensitivity indicating calamity

By Kudzai Jakachira

A major climate Earth’s sensitivity calculation published by Reviews
of Geophysics reveals how the world will ultimately warm if the
carbon dioxide in the atmosphere is preserved at double the level of
pre-industrial times.

While an exact figure is still not possible, low levels of warming are
now found to be far less likely than previously thought. Very high carbon dioxide levels are slightly less likely too.

There is much greater certainty that, if left unchecked, global warming would be high enough to bring very severe impacts and risks worldwide. The study, which was organised by the World Climate Research Programme
(WCRP) and involving many leading climate scientists, looks at a
measure called “equilibrium climate sensitivity”.

This refers to how much global average temperatures will increase by in the long-term following a doubling of carbon dioxide concentrations. It can be estimated using three main lines of evidence:
Temperature measurements made with thermometers from 1850 (when enough
global coverage began) to the near present.

By comparing temperatures,
CO₂ levels and the effect of other climate drivers in the past and present, we can estimate the longer-term changes.

Evidence from paleoclimate records from the peak of the last ice age 20,000 years ago, when CO₂ was lower than now, and a warm period
around 4 million years ago when CO₂ was more comparable to today. We can tell how warm the climate was and how much CO₂ there was in the
atmosphere based on the make-up of gases trapped in air in ancient ice
cores.

Present-day observations – for instance from satellite data – andevidence from climate models, theory and detailed process models that
examine the physics of interactions within the climate system.

Despite its importance, equilibrium climate sensitivity is very
uncertain and for many years the standard estimate has been 1.5°C to 4.5°C. In its 5th Assessment Report, the Intergovernmental Panel on
Climate Change (IPCC) gave these values as the “likely range”, which meant it considered there was at least a 66% chance that it fell within this range. Or, in other words, it judged there was up to a 33%
chance that warming would either be less than 1.5°C or more than 4.5°C.

The new study suggests that this “likely range” has narrowed to, at most, 2.3°C to 4.5°C – or possibly an even narrower range. The lower end of the range has therefore risen substantially, meaning that
scientists are now much more confident that global warming will not be
small.

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