Another record month for warming, as science looks way back for parallels

August 2016 was the world’s hottest month on record, according to the National Oceanic and Atmospheric Administration’s latest monthly report on global temperatures, but we seem to be setting such records all the time. So how hot is our planet these days compared to the historical record?

Reacting to the NOAA report, two prominent climate scientists said last week that current patterns are outside the normal range of conditions going back thousands of years at the least, and perhaps even 100,000 years.

Even those conjectures pale next to those of a new paper published in Nature, which concludes that the Earth hasn’t seen a warming trend of this size in at least 120,000, and perhaps 2 million, years.

The newest Global Analysis from NOAA confines its comparisons to the “period of record” that begins in 1880. It finds that last month’s combined average temperature over land and sea surfaces was the highest for any August in that 137-year period, and also set a record in the extent of its departure from the 20th-century average.

It was marked by a range of major anomalies and weather events around the world, mapped here, from heavy rains and flooding along the Mississippi River to the second-highest August temperature ever recorded in Bahrain, and the highest in parts of Asia.

Very hot but no outlier

But August was no outlier, not by any means. It marked “the 16th consecutive month of record warmth for the globe.” It also closed out a three-month period that, with an average land/sea temperature of 61.7 degrees Fahrenheit, was the highest for any June-July-August slice of summer in the post-1880 record.

While August’s departure from the 20th century average “surpassed the previous record set in 2015,” NOAA notes that “fourteen of the 15 highest monthly land and ocean temperature departures in the record have occurred in [the 19 months] since February 2015.”  (The 15th was in January 2007.)

OK, you might say — all those statistics make it sound like we’re setting temperature records all the time, so maybe we’re just having, like, a little global heat wave. Haven’t we heard that this year’s patterns reflect the fading El Nino? What’s the long-term context, anyway?

It’s just too tempting to recall that the last time Earth posted a month that was cooler than the global average, according to NOAA, President Ronald Reagan was celebrating Christmas and his re-election to a second term.

But it’s more to the point  to note that the NOAA report moved Michael Mann, the Penn State meteorologist who studies just that kind of question, to tell USA Today last week that “it is plausible that this summer was the warmest in thousands of years, perhaps even longer.”

There is now very robust paleoclimate evidence that the past decade was likely Earth’s warmest in more than a thousand years, and there is somewhat more tentative but nonetheless compelling evidence that we have moved into territory unseen in more than a hundred thousand years.

To which NASA climatologist Gavin Schmidt added his observation that the warming trend of recent decades “seems exceptional in many hundreds and perhaps thousands of years. Glacial retreat is indicative of this, since they are unearthing soil, debris, and trees that were buried 1,000 to 4,000 years ago.”

Looking back 2 million years

Even those are short-term observations compared to the very long view taken by Carolyn Snyder, who now works on climate policy for the U.S. Environmental Protection Agency but dug into ancient climate history while earning a doctorate from Stanford University.

The Nature paper that resulted, “Evolution of global temperature over the past two million years,” was published Monday and attempts to address a trio of deficits in our understanding of climate in the very distant past:

Actual temperature measurements don’t go back very far; indirect measures offer fairly limited opportunities to go back further; so efforts to “reconstruct” global temperature from other evidence has only been attempted “for a few isolated windows of time, [while] continuous reconstructions across glacial cycles remain elusive.”

Using a variety of “proxies,” such as the distribution of certain minerals and levels of ocean acidity detected in seabed cores, Snyder prepared temperature reconstructions for periods 5,000 years in length — some 400 of them, for a total time span of 2 million years. This was sufficient to cover two great glacial cycles, and also to demonstrate that the temperature trends she inferred tracked closely with measurements of carbon dioxide in the atmosphere.

Snyder concludes that the planet was cooling overall from until about 1.2 million years ago, then entered a period of relative stability for most of the next 800,000 years — up until the recent warming trend. An excellent synthesis from the AP’s Seth Borenstein:

Temperatures averaged out over the most recent 5,000 years — which includes the last 125 years or so of industrial emissions of heat-trapping gases — are generally warmer than they have been since about 120,000 years ago or so, Snyder found. And two interglacial time periods, the one 120,000 years ago and another just about 2 million years ago, were the warmest Snyder tracked. They were about 3.6 degrees (2 degrees Celsius) warmer than the current 5,000-year average.

More provocatively, she extrapolated from those findings to suggest that future warming will occur on a scale far exceeding the most dire forecasts under discussion in the scientific mainstream — up to 13 degrees Fahrenheit.

Forecast doubted, but not history

And that second point has brought considerable criticism from authorities like Michael Mann and Gavin Schmidt, referenced above.  The general objection seems to be that while Snyder’s reconstructions have some plausibility, as do the correlations with atmospheric CO2, her basis for extrapolating future temperatures is either unpersuasive or, for some critics, plainly flawed. 

“This is simply wrong,” Schmidt told National Geographic. And Mann told the AP that Snyder’s projections for the future were “so much higher than prevailing estimates that one has to consider it something of an outlier.”

But on Snyder’s reconstruction of the past, the climate science community seems generally supportive of the work and intrigued by the results — while calling for further work to test and refine her conclusions that current warming seems to represent the greatest the planet has seen in at least 120, and possibly 2,000, millennia.

* * *

The Nature paper, “Evolution of global temperature over the past two million years,” can be read here without charge.