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continental warming, a drift into ‘unknown lands’

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While our planet is warming globally, this warming is not evenly distributed over its surface. In particular, the rise in temperatures over continents is occurring at a rate almost twice that of the world average.

More precisely, between the periods 1850-1900 and 2006-2015, the air on the surface of the continents warmed by 1.5 ° C for an overall increase of 0.8 ° C. This figure is all the more significant given that land constitutes precisely the part of the globe on which we live and have developed our infrastructures as well as worked on our agriculture.

Comparison between the rise in temperatures on the continents (gray curve) and over the entire globe (black curve). The timescale runs from 1850 to 2018. Credits: IPCC / IPCC.

The thermal inertia involved?

The difference between continental and oceanic warming is often explained as being due to the greater thermal inertia of the latter. It is true that the sea air takes longer to heat up because the water it flies over has a greater heat capacity than the ground. In addition, the oceanic surface is stirred by the wind which diffuses the heat over a greater depth.

However, by considering only these factors, one arrives at the conclusion that at equilibrium, the warming of the air should be the same at sea as on land. Indeed, the greater inertia of the ocean only creates a time lag without changing the end result. However, models and paleoclimate observations show that even in a situation of equilibrium, that is to say when climate change ceases, the continents end with a thermal variation systematically greater than the oceans.

temperatures
Evolution of the distribution of continental temperature anomalies over the past seven decades. Credits: NASA / GISS.

Differential heating: the essential role of evaporation

In reality, the essential phenomenon which explains the asymmetry between the two types of surface is evaporation. When the available heat increases, part of the energy will be used to evaporate the water instead of changing the air temperature. In this context, the oceans present themselves as an infinite source of humidity. Conversely, continental surfaces will have an evaporation capacity quickly limited by the reserve available on the surface. In fact, the additional energy provided by greenhouse gases will be expressed mainly in temperature. Rather, above the seas, this energy will be used to evaporate water at an accelerated rate. It will therefore be expressed relatively little in temperature.

However, in return for this less oceanic warming we find a worldwide acceleration of the hydrological cycle, with all the resulting implications (more intense and / or frequent rains, reinforcement of tropical cyclones, etc.).

On the animation above, we show the Distribution annual temperature anomalies over continental surfaces and their sliding into unknown lands between 1951 and 2020. Two elements appear clearly on the computer graphics. On the one hand, the progressive shift of the curve to the right, corresponding to a general warming of the continental air. On the other hand, a widening of the distribution, the cold part passing less quickly to the right than the hot part. A characteristic which essentially testifies to more or less significant thermal rises according to the regions. Northern Canada is warming, for example, 4 to 6 times faster than the southeastern United States.



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