Unsettled: What Climate Science Tells Us, What It Doesn’t, and Why It Matters

by Steven E. Koonin

Yes, it’s true that the globe is warming, and that humans are exerting a warming influence upon it.

that the warmest temperatures in the US have not risen in the past fifty years.

Humans exert a growing, but physically small, warming influence on the climate. The deficiencies of climate data challenge our ability to untangle the response to human influences from poorly understood natural changes.

My frankness about the state of climate science was less popular in the scientific community, however. As the chair of a highly respected university earth sciences department told me privately, “I agree with pretty much everything you wrote, but I don’t dare say that in public.”

And a prominent defender of the idea that The Science is settled enough published a response to my Op-Ed that began by calling for New York University to reconsider my employment, went on to misrepresent many of the things I had written, but then, bafflingly, acknowledged that most of the uncertainties I’d mentioned were well known and much discussed among experts.

Climate alarmism has come to dominate US politics, especially among Democrats, where I have otherwise long felt most comfortable politically.

Some people argue that there’s no harm in a bit of misinformation if it helps “save the planet,” and indeed, when phrases like this (however unwarranted or inaccurate) are being used to describe the stakes, perhaps it isn’t surprising that some climate scientists are less than objective when talking to the public.

To do that we need to get some broad based support, to capture the

That, of course, entails getting loads of media coverage. So we have to offer up scary scenarios, make simplified, dramatic statements, and make little mention of any doubts we might have.

Within a few years] winter snowfall [in the UK] will become a very rare and exciting event. Children just aren’t going to know what snow is.” —DAVID VINER, SENIOR RESEARCH SCIENTIST, 200012 •“European cities will be plunged beneath rising seas as Britain is plunged into a Siberian climate by 2020.” —MARK TOWNSEND AND PAUL HARRIS, QUOTING A PENTAGON REPORT IN THE GUARDIAN, 200413

In a speech in which he compared human-caused climate change to weapons of mass destruction, then secretary of state John Kerry (now the Biden administration’s climate envoy) said, “The science is unequivocal . . . President Obama and I believe very deeply that we do not have time for a meeting anywhere of the Flat Earth Society.”18 But the science is not settled. Open debate is at the heart of the scientific process; it is absurd that scientists should fear being labeled antiscience for engaging in it. In that light, this book issues a challenge and solicits, indeed welcomes, informed argument and disagreement. This would be an important step toward wiser societal decisions on climate and energy—and less heat in the debate over the science of our warming planet.

In my experience, the reports largely do meet that expectation, and so much of the detail in the first part of this book, the science story, is drawn from them. But a careful reading of the most recent assessment reports also reveals some elementary failures that mislead or misinform readers on important points. What those failures are, how they came about, how the media promulgates them, and what can be done to correct them is another dimension of the science story.

In fact, the UN’s World Meteorological Organization defines climate as a thirty-year average, although climate researchers will sometimes discuss averages over a period as short as ten years.

More expert readers would know that temperature trends due to a changing global climate are smooth over much larger distances than the blobs on these maps—you know it, too: remember Hansen and Lebedeff’s 1,200 km? So how could New York City have warmed so much more rapidly than a region in central New York State 250 km (150 miles) away? Buried deep within the article one finds: Urban heat effects, changing air pollution levels, ocean currents, events like the Dust Bowl, and natural climate wobbles such as El Niño could all be playing some role, experts say.

Nevertheless, the growing ocean heat content is the surest indication that the planet has indeed been warming in recent decades.

As you can see, centuries of warmer temperatures gave way gradually around the year 1000, leading up to the Little Ice Age, an unusually cold period that ran from about 1450 to 1850. This was followed by more rapid warming that has continued until today.

has only low confidence that the global warming of the past thirty years has exceeded the range of reconstructed temperatures.

so AR5 has medium confidence that the last thirty years were likely (a two-out-of-three chance) the warmest thirty-year period of the last fourteen hundred years for the Northern Hemisphere.

Analyses of layers deep in these cores can tell us about the distant past—the oldest cores now go back almost three million years.

the most recent warm period before the present began about 127,000 years ago and lasted for some 20,000 years.

Carboniferous period extended from ~360 to ~300 million years ago, the interval between evolution’s invention of trees and its invention of tree rot.

We dedicated the wind turbines that evening, and the next morning we flew further south on a cargo prop plane to visit the Pole. The temperature was –33ºC (–27ºF); we stayed for eight hours. I was able to enjoy this amazing experience thanks to the insulation of the cold weather gear, which intercepted my body heat, impeding its flow away from me and into the surrounding air.

blocks almost 100 percent of those it does, adding more water vapor to the atmosphere won’t make the insulation much thicker—it would be like putting another layer of black paint on an already black window.

not true for carbon dioxide. That molecule intercepts some colors that water vapor misses, meaning a few molecules of CO2 can have a much bigger effect (like the first layer of black paint on a clear window). So the greater potency of a CO2 molecule depends upon relatively obscure aspects of how it, and water vapor, intercept heat radiation—another example of why the details are important when attempting to understand human influences on the climate.

Finally, the dashed black line shows an additional 0.8 percent loss of cooling power when the CO2 concentration is raised to 800 ppm, roughly twice what it is today; this change is barely visible in the sides of the large dip.

Second, although the effect of CO2 at today’s concentration is significant (7.6 percent), doubling it doesn’t change things much (an additional 0.8 percent) due to the “painting a black window” effect we’ve already discussed.

(For example, the earth was about 0.6ºC cooler during the fifteen months that followed the eruption of Mt. Pinatubo in June 1991.11)

Human influences today amount to just over 2 W/m2, or slightly less than 1 percent of that natural flow (about the same influence as half a cucumber on the daily human diet).

Also shown in Figure 2.4 is how uncertain we are about these various forcings. While the warming effects of CO2 and other greenhouse gases are known to within 20 percent, the uncertainty in the cooling influence of human-caused aerosols is much larger, making the total human-caused forcing uncertain by 50 percent—that is, the best we can say is that the net human influence today is very likely to be between 1.1 and 3.3 W/m2.

The fact that human influences currently amount to only 1 percent of the energy that flows through the climate system has important implications, and means there’s a lot to understand.

The simple fact that carbon dioxide lasts a long time in the atmosphere is a fundamental impediment to reducing human influences on the climate.

And as with CO2, today’s methane concentrations are dramatically higher than those of the past few million years, beginning a sharp rise about four thousand years ago.

Another difference is that a methane molecule lasts in the atmosphere for only about twelve years—though after that, chemical reactions covert it to CO2.

For instance, the 300 million tons of methane humans emit each year is only 0.8 percent of the 36 gigatons of CO2 emitted by burning fossil fuels. But as shown in Figure 3.2, that methane has a disproportionate warming influence, equivalent to ten gigatons of CO2.

Rather, most methane emissions arise from enteric fermentation (digestion in cattle—mostly emitted from the front of the animal, not the back) and other agricultural activities, particularly rice cultivation; the decay of material in landfills is also significant. So any effort to drastically reduce emissions must also address those sources.

There is no question that our emission of greenhouse gases, in particular CO2, is exerting a warming influence on the planet. Human influences on the climate have grown over the past decades and will continue to grow under all but the most radical scenarios for future emissions.

But another equally serious issue is also illustrated here: Figure 4.3 shows that the ensembles fail to reproduce the strong warming observed from 1910 to 1940. On average, the models give a warming rate over that period of about half what was actually observed. As the IPCC noted in measured and somewhat antiseptic language: It remains difficult to quantify the contribution to this warming from internal variability, natural forcing and anthropogenic forcing, due to forcing and response uncertainties and incomplete observational coverage.13 More bluntly, they’re saying that we’ve no idea what causes this failure of the models. They cannot tell us why the climate changed during those decades. And that’s deeply unsettling, because the observed early twentieth-century warming is comparable to the observed late twentieth-century warming, which the assessment reports attribute with “high confidence” to human influences.

The best-known example is that of El Niño events (technically, the El Niño-Southern Oscillation), a shift in heat across the equatorial Pacific Ocean that occurs irregularly every two to seven years and influences global weather patterns. A slower behavior that’s less well known is the Atlantic Multidecadal Oscillation

That the models can’t reproduce the past is a big red flag—it erodes confidence in their projections of future climates. In particular, it greatly complicates sorting out the relative roles of natural variability and human influences in the warming that has occurred since 1980.

Cloud-aerosol interactions

One of the reasons the climate sensitivity is so uncertain is that aerosols currently exert a cooling influence that partially offsets (or masks) the greenhouse gas warming.

Greenhouse gases alone caused a warming of 1.5ºC (2.7ºF) from 1900, which is partially offset by a cooling of about 0.6ºC (1.1ºF) from human-caused aerosols (solar variation and volcanic aerosols don’t have much long-term effect).

If “the uncertainties in modeling” mean these models can’t give us useful information about what albedo modification might do, it’s hard to see why they would be any better at predicting the response to other human influences.

event attribution studies

The bottom line is that the science says that most extreme weather events show no long-term trends that can be attributed to human influences on the climate.

We can all agree the globe has gotten warmer over the past several decades. Here’s another summary statement from the IPCC’s AR5: [S]ince about 1950 it is very likely that the numbers of cold days and nights have decreased and the numbers of warm days and nights have increased . . . there is medium confidence that globally the length and frequency of warm spells, including heat waves, has increased since the middle of the 20th century.10 The science pointing to longer or more frequent warm spells like heat waves inspires a lukewarm “medium confidence,” but that of a general trend toward warmer temperatures gets the IPCC’s “very likely” designation, meaning there is a mere 10 percent chance of it being mistaken.

The US government’s most recent assessment report, the 2017 Climate Science Special Report (CSSR), is not just misleading on this point—it’s wrong. I say that, to use the assessment reports’ lingo, with Very High Confidence because of some sleuthing I did in the spring of 2019. What emerged is a disturbing illustration of how non-experts are misled and science is spun to persuade, not inform.

shows that the average coldest temperature of the year has clearly increased since 1900, while the average warmest temperature has hardly changed over the last sixty years and is about the same today as it was in 1900.

But it sure seemed possible that the ratio of record highs to record lows shown in Figure 5.1 goes up not because record highs are becoming more common, but because as the coldest temperatures warm, the ratio’s denominator

I quickly understood that there’s a big problem with running records—they tend to become less frequent as the years go on because each new record “raises the bar” and makes it more diffi cult to achieve a subsequent record.