Eight cooler years cannot be extrapolated to draw conclusions on long-term global warming
Eight cooler years cannot be extrapolated to draw conclusions on long-term global warming
(Reuters) - At least several decades’ worth of data is needed to infer trends in global temperatures, climate experts told Reuters, and while short-term fluctuations might lead to several years that are cooler than a peak year preceding them, the longer-term trend is still upward.
Users online are sharing a post showing graphed temperature data over the past eight years from the National Oceanic and Atmospheric Administration (NOAA) alongside claims that the data mean the Earth is cooling despite ongoing greenhouse gas emissions.
The claim that “The last 8 years have witnessed global COOLING despite 450 billion tons of emissions worth 14% of total manmade atmospheric CO2” can be seen on Facebook (here) and (here). It originated on Twitter (here).
The circulating graph shows NOAA “Global Land and Ocean” temperature data from 2015 to 2022, with 2016 as the hottest year on record and yearly fluctuations after that adding up to an average decrease of about one tenth of one degree Celsius over the decade. The graph was published in January on the website of the NOAA National Centers for Environmental Information (here).
However, climate experts from NASA and Berkeley Earth, an organization focused on environmental data science (berkeleyearth.org/about/), told Reuters that data from short periods of time showing cooling temperatures cannot be extrapolated to longer-term trends and do not prove that long-term global warming is a hoax. Moreover, they said, the warming effect of carbon emissions is cumulative and would not be expected to track with yearly fluctuations.
SHORT TERM FLUCTUATIONS
Global surface temperatures are heavily influenced by annual Pacific Ocean patterns known as El Nino and La Nina, and collectively as ENSO (El Niño–Southern Oscillation).
During an El Nino event, there is a discharge of the heat, or evaporation, from the ocean into the atmosphere, Chris Colose, a climate scientist at NASA’s Goddard Institute for Space Studies (GISS), told Reuters. “This results in hotter-than-average temperatures globally.”
In contrast, during La Ninas, when there are unusually cooler temperatures, heat is redistributed from the atmosphere to the ocean, Zeke Hausfather, a climate scientist at Berkeley Earth, told Reuters. “Short-term year-to-year variability is strongly influenced” by El Ninos and La Ninas, he said.
A large El Nino event followed by a La Nina can “lead to a temporary ‘pause’ in global temperatures over timescales of a decade or so”, Hausfather said. This is “what we are now seeing after the 2015/2016 super El Nino event”.
The short-term fluctuations producing periods of several years where temperatures appear to plateau or even cool create an illusion, said Robert Rohde, Berkeley Earth’s lead scientist, which is illustrated in a graphic that he calls “The Staircase of Denial,” showing temperature averages still climbing over the longer term (here).
A similar graphic called “The Escalator” from Skeptical Science, a science education organization, can be seen.
Reuters has previously debunked claims that a local slowdown in Greenland’s ice melt, which is influenced by ENSO fluctuations, suggests that human-driven warming on a global scale isn’t happening.
LONG-TERM TRENDS
No change, or even a cooling trend, “can be found in short periods of time amid an overall longer positive trend,” John Bateman, a NOAA meteorologist, told Reuters. In fact, because of short-term fluctuations, selecting different narrow slices of the same timeframe can even show opposite patterns.
While the NOAA “Global Land and Ocean” temperature data from 2015 to 2022 do show a slight drop in average annual surface temperatures after 2016, the hottest year on record, that can be explained by ENSO fluctuations, Bateman said - 2016 “started off with a strong El Nino, which helped boost global temperatures to record highs”, and since then, there have been about three La Ninas.
When the effects of ENSO are removed from the record, Hausfather said, the apparent pause in warming disappears, which Hausfather has illustrated in a series of graphs posted to Twitter.
Whenever there is a trend with variability like global temperatures, “you can isolate cherry-picked intervals and claim that something has paused or accelerated, but this is not appropriate”, Colose said. “The Earth will continue to get warmer, and this will always be evident on the multi-decadal window, not necessarily year to year.”
EMISSIONS HAVE CUMULATIVE EFFECT
The source of the original claim told Reuters that he stands by it, reiterating in an email that “It makes no sense that 450+ billion tons of emissions worth 14% of total manmade CO2 have failed to cause any warming,” and therefore that the “notion that CO2 warming drives global temperature can only reasonably be viewed as a hoax.”
However, the warming effect of carbon emissions is cumulative, Hausfather told Reuters. “The effect of CO2 on the climate is a function of cumulative emissions given the long lifetime of a perturbation of atmospheric CO2 (~500k years for full removal by sinks and geologic processes),” he said, adding that a constant increase in temperature year over year is not expected because “surface air temperatures are subject to large short-term natural variability from things like volcanic eruptions and La Nina or El Nino events.
VERDICT
Misleading. Temperature data for the past eight years do not reflect long-term trends, experts say, and longer-term data clearly show a continuing rise in overall global temperatures. Cumulative effects of CO2 emissions would not be reflected in yearly temperature variations.
