James Hansen
April, 2020. This year, 2020, should have record global warmth according to widespread media reports in April. The reports were based largely on a NOAA conclusion that such a record was likely with 75% confidence. April has since come in with record warmth for the month, although practically the same as April 2016. That should seal the deal, right?
Not so fast. Their expectation is based on the fact that the first few months of 2020 are almost as warm as the same months in 2016, and the fact that global temperature fell rapidly in the last eight months of 2016, as the super El Nino of 2015-16 faded and was replaced by a La Nina in the tropical Pacific Ocean.
The graph shows the Nino temperature index, including the NOAA NCEP model projection, which predicts a rapidly developing La Nina this year. So, the 2020 global temperature may fall as fast or faster than in 2016. A strong La Nina, if it occurs, will affect 2021 as well as the rest of this year, in which case we do not expect record annual global warmth until 2022, at the earliest.
The game of predicting near-term global temperature records is of little import. We just want to insure against public misinterpretation, if, as is perhaps probable, 2020 does not achieve the predicted record.
Tropical ENSO (El Nino Southern Oscillation) variability is the largest cause of inter-annual variability of global temperature, but there are other factors. The increase of greenhouse gases, mainly CO2 and CH4, will give 2020 a warming boost, but that will be partly offset by the present deep minimum of solar irradiance. The wild card is caused by a reduction of human-made aerosols, due to reduced emissions during the ongoing global Covid-19 epidemic. Reduced aerosols will cause a boost in warming, but unfortunately global high-quality aerosol measurements are not being obtained.
May 2020 was the warmest May since adequate global data began in 1880, exceeding the next warmest May (2016) by 0.060C. Global surface temperature was 1.020C relative to the 1951-1980 base period and 1.290C relative to 1880-1920.
Siberia continued to be unusually warm, with a large area more than 40C warmer than during the 1951- 1980 base period. However, large portions of the United States, Canada, Europe and Australia were cooler than they were in the 1951-1980 average.
The first five months of 2020 are the 2nd warmest January-May in the record at 1.160C relative to 1951- 1980.
The last seven months of 2016 were relatively cool, aided by a shift from El Niño to La Niña conditions in the tropical Pacific, so 2020 has a chance to be the warmest year. Thus, there were widespread media reports that 2020 likely would be the warmest year, based mainly on a NOAA prediction.
Our update last month suggested caution with that prediction, because of strong evidence that 2020 is also headed into a La Niña. The research group predicting El Niño and La Niña has become notoriously conservative, almost waiting until one is in place before “predicting” it, but the NCEP model for several weeks has been consistently predicting a rather strong La Niña.
In a companion Communication we discuss what might be learned at the end of the year from comparison of 2016 and 2020 in their race for the title of the warmest year.
Supporting graphs available at https://csas.earth.columbia.edu.
James Hansen, former director of the NASA Goddard Institute for Space Studies, is director of the Climate Science, Awareness and Solutions program at the Columbia University Earth Institute.
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