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Evidence that global evapotranspiration makes a substantial contribution to the global atmospheric temperature slowdown

Leggett, L. Mark W., Ball, David A.
Theoretical and applied climatology 2019 v.135 no.1-2 pp. 649-675
air temperature, biosphere, carbon dioxide, evapotranspiration, heat, models, regression analysis, surface temperature, time series analysis
The difference between the time series trend for temperature expected from the increasing level of atmospheric CO₂ and that for the (more slowly rising) observed temperature has been termed the global surface temperature slowdown. In this paper, we characterise the single time series made from the subtraction of these two time series as the ‘global surface temperature gap’. We also develop an analogous atmospheric CO₂ gap series from the difference between the level of CO₂ and first-difference CO₂ (that is, the change in CO₂ from one period to the next). This paper provides three further pieces of evidence concerning the global surface temperature slowdown. First, we find that the present size of both the global surface temperature gap and the CO₂ gap is unprecedented over a period starting at least as far back as the 1860s. Second, ARDL and Granger causality analyses involving the global surface temperature gap against the major candidate physical drivers of the ocean heat sink and biosphere evapotranspiration are conducted. In each case where ocean heat data was available, it was significant in the models: however, evapotranspiration, or its argued surrogate precipitation, also remained significant in the models alongside ocean heat. In terms of relative scale, the standardised regression coefficient for evapotranspiration was repeatedly of the same order of magnitude as—typically as much as half that for—ocean heat. The foregoing is evidence that, alongside the ocean heat sink, evapotranspiration is also likely to be making a substantial contribution to the global atmospheric temperature outcome. Third, there is evidence that both the ocean heat sink and the evapotranspiration process might be able to continue into the future to keep the temperature lower than the level-of-CO₂ models would suggest. It is shown that this means there can be benefit in using the first-difference CO₂ to temperature relationship shown in Leggett and Ball (Atmos Chem Phys 15(20):11571–11592, 2015) to forecast future global surface temperature.