Scientists have been quietly gnashing their teeth in frustration over deniers’ latest attempts to pervert their research on global warming. Until now.
In an article published in the latest issue of the journal Geophysical Research Letters (sub. required), David R. Easterling of NOAA’s National Climatic Data Center and Michael F. Wehner of the Lawrence Berkeley National Laboratory set out to refute the “global cooling” lies peddled by Fox News, The Washington Post, and the Republican Party once and for all.
They start off by pointing out that periods of no trend or even slight cooling in the globally averaged surface air temperature record have been quite common in the last 34 years and in climate model simulations of the 20th and 21st centuries so the most recent trend is nothing new. In fact, given the natural variability inherent in the climate, they say that it is highly likely that we will experience several more periods of a decade or two where there is no trend, or slight cooling, “in the presence of longer-term warming.”
The myth of “global cooling” has largely been perpetuated by deniers who zealously cherry pick the data, fitting linear trend lines to particular decadal temperature records, to “reinforce their point of view,” the authors say. Indeed, deniers often single out the 1998 – 2008 period, during which there was no significant trend, to argue that global warming has stopped, for instance.
That’s a red herring because one could just as easily fit the trend to the 1999 – 2008 period and obtain a statistically significant positive trend (1998 having been an unusually warm year because of the strong El Niño).
Or take the 1977 – 1985 and 1981 – 1989 periods: Looked at individually, they both show no trend but, when embedded in the 1975 – 2008 period, they become part of a larger, substantial warming trend (the top left graph in the image is a close-up of the 1977 – 1985 period and the top right one is a close-up of the 1981 – 1989 period).
While it may seem easy to fixate on these short-term trends, there’s a very simple reason for why scientists do not do so: they don’t matter. Easterling and Wehner:
“Climate scientists pay little attention to these short-term fluctuations as the short term “cooling trends” mentioned above are statistically insignificant and fitting trends to such short periods is not very meaningful in the context of long-term climate change.”
To further disprove the deniers’ claims, the authors ran a series of climate model simulations to look for both positive and negative decadal trends in the projected temperature record for the 21st century. One of their simulations, while predicting an overall warming trend of about 4°C by the end of the century (in line with most IPCC models), showed that the 2001 – 2010 and 2016 – 2031 periods would not see a clear trend, or, at most, see a small, statistically insignificant cooling trend.
Because the model was run without any simulated volcanic eruptions or solar variability (which could result in prolonged periods of cooling), Easterling and Wehner attributed the observed trends entirely to natural internal variability.
Overall, they found that there was about a 5% chance of a negative decadal trend occurring during the 21st century; if they restricted their timeline to only the first half of the 21st century, the probability of a negative decadal trend rose to 10%.
Their simulations also demonstrated that, as greenhouse gas forcing increases, the percentage of statistically significant positive trends increases – from 26% for the first half of the century to 47% for the entire century – while that of statistically significant negative trends decreases.
They conclude by stating the obvious: though the natural variability of the climate will likely produce several more periods of slight cooling or no change, the overall trend is one of significant, and sustained, warming.
“Claims that a global warming is not occurring that are derived from a cooling observed over such short time periods ignore this natural variability and are misleading.”