8 March 2017
First of all, this was a rare event and rare events happen, rarely even in a non-warming climate. In 1900 you would expect a February this warm about every 160 years. According to the study:
Around 1900 the return period would have been about 160 years, i.e., a probability of 0.6%, with an uncertainty from 0.05% to 1.4% each year. Thus, on the basis of the observed trend we find that the probability has increased by roughly by a factor of 13. We have 95% confidence that the increase in likelihood of events like February 2017 is at least a factor four. A similar fit can also give the return period of the 1954 temperature. At that time it was a very unusual situation with a return period of roughly 1 in 200 year (about 0.5% chance). That also means, however, that the odds of finding such an event in 120 years of data are about 50-50, just like throwing a 6 in three throws of dice. The weather maps also confirm that February 1954 had much more unusual weather than February 2017, while the circulation for this year is not as unusual, adding support to the fact that the heating trend has made high temperatures like last month more commonplace than they were in the 1950s.
Using climate models, they then determined how often we should see this kind of warmth now with higher greenhouse gases and much warmer oceans:
The CMIP5 climate model simulations (Figure 4) show an 18 year return period for Februaries over the CONUS as warm or warmer than 2017, similar to the observational estimate. Without the historical anthropogenic increases in greenhouse gases, the “best estimate” of the return period in the models is around 60 years, or a tripling of its probability — the lower bound on the 95% range of the probability change is 1.8x now from past greenhouse gas increases. The model estimate of the greenhouse-driven increase in warm February probability is within the uncertainty range of the observed increase in probability, though on the lower range of the observational estimates. Around 2050 temperatures like this are projected to be completely normal, occurring approximately every three years on average.
The paper’s conclusion summarises the main points well:
Observations show a trend in February CONUS temperatures of 1 to 2.5 times the global mean temperature. This has increased the probability of a temperature like the one observed in 2017 from 1 in 160 to 1 in 12 years, a factor of 13 larger (the observations show that this factor is larger than four with 95% confidence). An even warmer February in 1954 was, at the time, a very rare event (probability about 0.5% per year), but this kind of temperatures have become almost normal by now.
Results from global climate models are inconsistent with the hypothesis that the increase in odds of warm Februaries was caused by natural forcing agents such as solar activity, which has declined since the 1960s, and volcanic eruptions. Meanwhile, the model results indicate that past historical increases in greenhouse gases have raised the odds of warm Februaries in the CONUS considerably. The observed trend is compatible with the effects of anthropogenic emissions of greenhouse gases. Since past and projected future greenhouse gas increases will continue to raise the temperatures, the frequency of winter months like February 2017 should be expected to increase over the coming decades.
The “too long did not read” version. Yes, a warm February can happen without the climate changing. It’s just now MUCH more likely to happen, and in 33 years, it will be almost a normal occurrence.