Why climate change is such a killer? Because disruption is never a linear process. We see that with ecosystem disintegration – where beyond a certain threshold cascading damage sets in – leading to shifts and collapse scenarios that are easy to succumb to, yet almost impossible to predict.

We also see this with weather extremes. A seemingly innocent shift in the climatic average (precipitation, or temperature – likely both) leads to an ‘unexpectedly’ [unless of course your name is Thomas Bayes or Carl Friedrich Gauss – then this would be intuitive we guess] large increase in the incidence of the old category of weather extremes – and a submerging new category of equally rare but now far extremer weather extremes. These extremes of course are responsible for a lot of the (ecological, agricultural, economical) damage of the (average) change.

We’ve done our bit of homework on the increase of weather extremes here at Bits of Science. The above normal distribution model is nice as an illustration, but reality is likely a bit different. Skewed to be more precise. That has to do with the very subject we’d like to discuss in this article: the influence of geography.

Geographical distribution of warming – climate sensitivity range is several degrees

There are many other factors why a seemingly insignificant average warming has killing consequences. An important one that’s also easily overlooked is the distribution of that average warming itself – distribution across a geographically diverse planet.

To add some numbers: say the Earth’s oceans (in this century) warm by between 1.5 to 2 degrees, then inland Africa, the Amazon, North America and much of Eurasia will warm twice as fast. Yet the Arctic Ocean has in turn about twice the average climate sensitivity of the Earth’s land masses. The same goes for high mountain ranges and other places where Earth likes to stores glaciers and ice caps. An inconvenient truth. An unfortunate truth. (And even a still underestimated truth – as those geographical differences are the result of positive feedbacks that climate models are generally still very conservative about. Reality could very well be worse than IPCC projections.)

All in all it’s a reason to never underestimate the huge significance of attempts to lower net global average warming – like that almost 0.5 degrees rise in expressed global ambition following from the UN climate conference in Paris, where countries agreed on pursuing the more ambitious 1.5 degrees target (although countries’ coupled pledged emission targets still lead to about 3 degrees!).

The world we will live in, depending on (political) choice. Shown above are two of the four main greenhouse gas concentration trajectories of the last IPCC report – the most ambitious one (RCP2.6) and one that is more like business as usual (RCP8.5), a world without real international cooperation on climate mitigation, a world that continues to burn fossil carbon reserves to meet the energy demands of a growing global population – in one word: Disaster.

IPCC’s best case: continents warm 1.5 degrees, Arctic warms 2 to 4 degrees

Under IPCC AR5’s most ambitious RCP2.6 greenhouse gas concentration trajectory the Arctic Ocean is forecast to warm by about 2 to 4 degrees Celsius between 2080 and 2100. Much of the continental masses of Africa, America, Eurasia and Australia will stay close to 1.5 degrees of warming – in that same scenario, one that shows a gently declining greenhouse gas concentration between 2040 (where it peaks just above 450 ppm CO2eq) to 2100 (where it will rest at around 430 ppm). Due to their large thermal mass, most of the oceans in this scenario will have warmed less than 1.5 degrees between 2080-2100.

(So Arctic warming is on average about twice as high as average land warming of inhabited areas.)

IPCC’s worst case: continents warm ~4.5 degrees, Arctic warms ~11 degrees

Under IPCC AR5’s worst case RCP8.5 greenhouse gas concentration trajectory the Arctic Ocean is forecast to warm anywhere between 9 and possibly 13 degrees Celsius by the end of this century. In that same scenario, that shows greenhouse gas concentrations rising for the rest of the century [so with a lot of additional warming in the 22nd century!], Earth continental masses show a relatively large lateral spread, with average warming between 3 (along coastlines) to 6 degrees (further inland). Ocean surface warming, a very inert process, is already between 2 to 3 degrees in this scenario – with of course a lot more to come as even the atmospheric greenhouse gas is still rising by 2100.

(So Arctic warming in this scenario too would be at least twice as high as average continental warming, more likely 2.5 times as high.)

A 3 degrees geographical spread in climate sensitivity, across latitudes(!)

Of course all such warming scenarios also depend on assumed climate sensitivity – climate sensitivity that still has its own spread of uncertainty [although significantly smaller for Bitsofscience.org regulars, see for instance our special climate sensitivity experts survey].

Shown below is a NASA GISS plot that shows geographical distribution of equilibrium climate sensitivity, for a doubling of atmospheric CO2 – with below that a coupled average climate sensitivity spread across Earth’s different latitudes.

Geographical distribution of equilibrium (100-yr response) climate sensitivity according to NASA GISS model runs, clearly showing Arctic amplification of global average warming.

NASA GISS shows a 3 degrees geographical spread in climate sensitivity, across latitudes. Notable is the smallest projected temperature rise around -60 degrees latitude at the southern hemisphere, a band of Earth that is dominated by ocean surfaces with very little land masses. Here thermal climate inertia is relatively large, therefore warming is very slow. The highest temperature rise happens at high latitudes at the northern hemisphere (Arctic) followed by an also clearly above-average temperature rise at high southern hemisphere latitudes, around Antarctica.

© Rolf Schuttenhelm | www.bitsofscience.org