Climate Sensitivity

There is no simple formula for how fast Earth is hotting up. If there were, 2 key components in the equation would be greenhouse gas emissions and temperature sensitivity to those emissions. Those 2 factors suggest that Earth is racing toward hellish heat.

The Last Glacial Maximum (LGM) was the most recent ice age, peaking roughly 25,000 years ago. Vast ice sheets covered much of what is now temperate lands.

Deglaciation from LGM happened rather abruptly around 14.5 thousand years ago – a geological period known as the Late Glacial Interstadial (LGI).

“Palaeoclimatologists have long sought to refine estimates of temperature changes during the LGM, as both a benchmark for climate models and a constraint on Earth’s climate sensitivity,” reports American paleoclimatologist Jessica Tierney. What Tierney found was a “climate sensitivity of 3.4 degrees Celsius.” What that means is the 3.4 degrees Celsius of planetary average surface air temperature change is expectable for each doubling of CO2 emissions.

There are some caveats to that number. First is that the climate sensitivity “is unlikely to remain constant across climate states. Palaeoclimate and modelling evidence suggests that it scales with background temperature, with lower values during cold climates and higher values during warm states.” In other words, 3.4 is likely to be a conservative estimate for planetary warming, and the sensitivity amplifier is likely to rise as it gets hotter.

Some known facts about global warming color the picture going forward. One is that warming is not uniform globally. The polar regions are warming much faster than the global average by a factor of 3 or more.

Another fact is that the oceans have absorbed ~95% of the initial warmth resulting from greenhouse gas emissions. There’s a daily dynamic to this. The Sun warms the atmosphere during the day and the seas soak up the heat during the night, as the atmosphere cools. Water is a better thermal store than air, so the oceans tend to retain the heat, returning the warmth to the atmosphere as a complex equilibrating process that involves ocean currents.

What is also known is that global warming is self-reinforcing. Another way to put that is that it is getting hotter faster – formally stated: nonlinear acceleration.

This is most apparent in deglaciation. Ice sheets are melting faster now than they were even a few years ago. And that’s not just because the ice is at the poles (though it helps). The melting is hastening and will increasingly accelerate. Sea level rise will quicken.

Terrestrial stores – from tundra to forests to swamps – are giving up their carbon, increasingly emitting greenhouse gases. The forest fires which have raged in Siberia, Australia, and North America – themselves a result of global warming – accelerate carbon release. From this alone warming worsens with greater rapidity.

Ocean warming is wreaking havoc directly and indirectly. The hot water is killing the coral reefs, which are home to 25% of the world’s fish, and the food source for even more deep-sea fish. By 2030 there won’t be any more coral reefs. Seafood, which is some 15% of the world food supply, is going bye-bye this decade.

Indirectly, the change in ocean currents and atmospheric conditions from warming are making more damaging storms. While not an amplifier of warming per se, along with wildfires, fiercer storms makes living on land increasingly hazardous.

Let’s get back to what 3.4 climate sensitivity means: for each doubling of carbon emissions expect it to get 3.4 °C hotter – on average – globally. Now, how to apply the amplifier.

1880 is the standard date cited for global warming temperature. Current global temperature is 1.4 °C warmer than then.

There’s a curve ball to global warming that helps figure out what’s expectable. Air temperature lags ocean rise by about 40 years (remember the bit about the oceans being the great thermal sink). The air temperatures we are experiencing now reflect emissions 4 decades ago, in 1980.

Since 1980, greenhouse gas emissions have doubled. That means that 40 years from now – in 2060 – it ought to be at least 3.4 °C hotter. Average global air surface temperature will rise – at a minimum – to 4.8 °C above the 1880 benchmark.

The thermalization and destabilization of the global biosphere are only going to accelerate. Considering what’s coming in the next few decades, covid-19 is a just a howdy-do to the impending apocalypse.


Jessica E. Tierney et al, “Glacial cooling and climate sensitivity revisited,” Nature (26 August 2020).

Ishi Nobu, The Fruits of Civilization (2019).

Several of Ishi Nobu’s writings in the Environment section of his weblog.