Everybody talks about the weather. And when we talk about how it feels like it’s changing, it seems to me, we talk about two aspects more than any other: how the storms are getting bigger and the seasons just aren’t what they used to be.
Still, human memory of weather is notoriously vague, even when it comes to those searing summers, flash-freezing winters, freak downpours and tree-toppling windstorms. Television meteorology seems to suggest that records are being broken all the time (maybe that’s always been so?).
And then we have the recurrent disclaimer of climate science: Yes, greenhouse gases are causing the world to warm and the climate to morph, but tying any particular change back to this underlying pattern, well, that remains a challenge.
But now comes a new paper published last Friday in the prestigious journal Science. Driven by satellite-enabled analysis, it establishes that key seasonal patterns are changing around the globe. And these changes are driven by human consumption of fossil fuels. What’s more, temperature shifts are not limited to the earth’s surface and those few meters of air above it, which is the zone where most statistics are gathered; the shifts show up throughout the atmosphere’s lowermost, 10-mile-thick layer known as the troposphere.
Most significant, perhaps: These environmental changes can be specified, and distinguished from patterns of natural variability, at a level of precision analogous to fingerprint analysis.
The study team, led by Benjamin Santer at Lawrence Livermore National Labratory, includes his colleagues Stephen Po-Chedley, Mark Zelinka, Ivana Cvijanovic, Celine Bonfils and Paul Durack; Qiang Fu from the University of Washington; Jeffrey Kiehl from the University of California, Santa Cruz; Susan Solomon from MIT; Cheng-Zhi Zou from the University of Maryland, and Carl Mears and Frank Wentz from the research company Remote Sensing Systems.
Among the climate experts admiring their work is John Abraham at the University of St. Thomas, a climate expert whose views I’ve shared here before. A commentator on climate matters for the UK Guardian, he describes the research team as “literally the best of the best climate scientists studying Earth’s atmosphere.” As for their findings, he writes,
Much of the fingerprint work that has been done over the last 30 years looks at broad-brush changes in annual climate. The focus on climate fingerprinting with the changing seasons is relatively new and makes this paper a significant contribution.
Not the temps but the swing
The key measurement in the paper is what the authors define as the amplitude of change between temperature readings taken in the warmest and coldest months of the year, as measured at a great many points around the globe.
The values in these paired readings don’t matter as much as the gap between them. If the annual high and low rise or fall in lockstep, amplitude is unchanged. If they move in different directions, or at different speeds, the amplitude grows. And as it grows, the cycle of the seasons changes.
In general, the researchers found that temperatures have been climbing more in summer than in winter (or spring and fall, for that matter), but the changes were not uniform around the world. Amplitudes grew faster in the northern hemisphere than the south, because there is much more land relative to sea surface above the equator than below, with the exception of the very high latitudes of the Arctic.
(The land-to-water ratio matters because fields and forests and pavement absorb more heat than the reflective sea surface, except when covered in snow. Scientists call the overall reflectivity of a planetary surface its albedo, and while the ocean’s contribution is fairly steady year round, the land surface in snowy regions lowers Earth’s albedo in summer and raises it in winter.)
Of the global region that includes Minnesota, says an announcement of the findings prepared by Livermore:
Away from the moderating effects of oceans, mid-latitude regions of Northern Hemisphere continents have a large seasonal cycle of atmospheric temperature, with frigid winters and hot summers. Satellite temperature data are consistent with models that project that this seasonal “heartbeat” is becoming stronger with human emissions of carbon dioxide.
What this means is that while both summers and winters are getting warmer, the extra warmth is larger in summer — rather opposite the fond hopes of those Minnesotans who can still be heard to say that global warming won’t be so bad if it takes some of the edge off February, while leaving July more or less alone.
Global date without gaps
In addition to covering the entire planetary troposphere, the researchers worked with an unusually long time frame — the nearly four decades from 1979 to 2016 — and satellite-derived data sets; though these don’t go back as long as measurements from weather balloons and surface instruments, some of which have been in use for 150 years, the paper notes that “satellite measurements of tropospheric temperature have near-global coverage and no gaps in time.”
Of the six global regions considered in the analysis, only one showed an overall cooling trend — the south polar region, which lies below the latitude 60o S. It’s a different story at the other end, above 60o N, where the region is warming and the amplitude of seasonal variation is shrinking, because of the well-known and unfortunate trends in the Arctic’s sea ice. From the paper:
Poleward of 60°N, all satellite datasets have substantial decreases in [amplitude]… This decrease arises in part from greater warming in Arctic winter than in Arctic summer. At the surface, greater winter warming is primarily related differences in the seasonal timing of feedback associated with sea ice retreat.
The ice-albedo feedback yields greater summertime heat storage in the Arctic Ocean, which in turn leads to increased wintertime sea ice retreat and increased wintertime heat release from the ocean to the polar atmosphere. This seasonality in sea ice trends and ocean heat storage is accompanied by seasonal changes in cloud and water vapor feedbacks and in ocean and atmospheric heat transport.
As for the mid-latitudes, including our region, the warming trend is more evident than its mechanics. In an accompanying assessment of the findings, the climatologist William J. Randel of the National Center for Atmospheric Research in Boulder, Colorado, writes that
The specific mechanism leading to enhanced tropospheric summertime warming is not well understood. Santer et al. suggest that surface-temperature changes are linked to summertime continental drying, with the resulting effects on water vapor amplifying changes at higher altitudes. This hypothesis will need further verification.
A key aspect of an explanation will need to address the larger and more extensive changes observed in the Northern compared to the Southern Hemisphere. Santer et al.’s findings provide further markers of a substantial human influence on Earth’s climate, affecting not only global averages but also local and seasonal changes.
As global satellite datasets lengthen in time and cover more parameters, we may expect identification of additional aspects of climate changes in the observational record, including regional and seasonally varying patterns in temperatures and other quantities. It is of crucial importance that the continuity and high quality of satellite observational records are maintained, especially for temperature, water vapor, and precipitation.
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The entire paper, “Human influence on the seasonal cycle of tropospheric temperature,” can be downloaded here, and the Randel commentary here, but access is not free.
