It’s remarkable to read that a new study suggests that some, or perhaps most, of any warming that has happened in the last 20 years may have been natural. And it’s remarkable not so much for the content, important as it is, as for the source. As David Whitehouse underlines at Net Zero Watch, it comes from the University of Oxford and the U.S. National Center for Atmospheric Research, via the Journal of Climate. These sources are not “deniers”. They are simply scientists who are doing honest research on the very large uncertainties in our understanding even of what has happened let alone why. But of course if the science is not “settled”, and if a significant share of whatever recent temperature changes have occurred might be natural, then it follows that a significant portion of the actually minor increase since 1850 might also be natural and CO2 not very important. And we say if the taboo on discussing this possibility is broken, real science can resume.
The piece is, typically, well-nigh unreadable, on the sound biological principle employed by creatures from skunks to monarch butterflies that being inedible deters predators. (No really; monarchs taste so bad it makes birds hurl, as does the viceroy long mistakenly thought to have sneakily imitated the colouration without wasting resources on the foul flavour but which is actually engaged in mutual “Mullerian mimicry”. But we digress.) And the headline “Is Anthropogenic Global Warming Accelerating?” ticks the right boxes, from there being man-made warming to something ominous happening.
We often make fun of the credentials of journalists who cover climate, partly because of the jibe hurled selectively at anyone who expresses skepticism that they’re “not a climate scientist” and partly because many of them really can’t make head or tail of prose like this:
“Estimates of the anthropogenic effective radiative forcing (ERF) trend have increased by 50% since 2000 (from +0.4 W m−2 decade−1 in 2000–09 to +0.6 W m−2 decade−1 in 2010–19), the majority of which is driven by changes in the aerosol ERF trend, as a result of aerosol emissions reductions. Here we study the extent to which observations of the climate system agree with these ERF assumptions.”
The conclusion appears to be “Situation complex, send more grants”. As in:
“Short-term ERF trends are difficult to verify using observations, so caution is required in predictions or policy judgments that depend on them, such as estimates of current anthropogenic warming trend, and the time remaining to, or the outstanding carbon budget consistent with, 1.5°C warming. Further systematic research focused on quantifying trends and early identification of acceleration or deceleration is required.”
As we’ve said before, it’s a bit of a letdown and also a bit self-serving to say that the result of this grant is we need another one. But it’s also refreshing, especially in an era where “publish or perish” has led to a serious crisis of overhyped and unreproducible results, to have some scientist or scientists say essentially we don’t know what it means.
To revert to form, we now add that we honestly doubt some of this language means anything. For instance “Successive IPCC reports have given assessments of the level of anthropogenic global warming, but no equivalent assessment of the rate of human-induced warming has been made.” What separates “anthropogenic” from “human-induced” we cannot begin to guess, and the level of man-made warming at various points in time surely gives even non-specialists some idea of the rate of it. (See, you divide the total difference by the number of time units and…)
Also, as Whitehouse correctly complains, the authors note that there was a hiatus in the first decade of the 21st century before asserting that warming resumed around 2010 without mentioning the natural-variability contribution of a series of El Niño events. (The specific passage being “Global temperatures show a clear change in trend between 2000 and 2020, characterized by temperatures remaining stable at around +1.0°C above preindustrial levels in the first decade, whereas in the second decade temperatures increase rapidly (with the rate of warming peaking at over +0.3°C decade−1).”)
Still, when you consider the drumbeat of non-specialists saying the science is settled, it takes some courage to say no, it’s not. And to say, however obscurely, that reductions in aerosol pollution, widely held to have a net cooling effect, might explain any warming in recent years. Because if so, the increase in CO2 does not explain it. And yet there has been an increase in CO2. So what’s going on?
Don’t ask these authors. They describe a large number of scenarios in which GHGs, aerosols or natural variability could be the main factor. And in doing so they make the key point that you can fit various regression analyses equally well to the actual data simply by employing “alternative forcing trend change assumptions”.
Eventually, if you hang in there, you get this passage: “Satellite observations and CMIP6 models agree that a relatively small AOD trend change has occurred over the last two decades, despite significant reductions in anthropogenic aerosol emissions in the Northern Hemisphere.” AOD being “aerosol optical depth”. Which means we’re not sure what the data are or how they relate to one another. Their final sentence is “Short-term ERF trends are vital to accurately assess this decade’s warming rate, with tangible, real-time impacts for global mitigation policy.”
ERF is, of course, “Effective Radiative Forcing”. But it’s not necessarily the case that those trends are vital, because of the point about the need “to accurately assess this decade’s warming rate”, which is really a matter of proper temperature measurements rather than calculations of how much humans would be warming the planet if conventional modeling assumptions about the impact of GHGs are correct which is, actually, the point in dispute.
Spelunking the prose, Whitehouse emerges with this key chunk: “although recent GMST trend changes may originate from anthropogenic ERF trends in isolation, PDO and ENSO signals covary substantially over the period, meaning credible counterhypotheses include a smaller aerosol ERF trend change assumption since 2000 with GMST trend change arising largely from internal variability.”
GMST is of course “Global Mean Surface Temperature”. So in English it might be that changes in temperature trends in the last 20 years are due to natural causes. Which, if true, puts the kibosh on the whole CO2-as-the-control-knob-of-the-global-thermostat theory. Alarmists are pretty slick with their explanations of why warming causes cooling, and snow proves it’s getting hotter, and drought or flooding does too. But at some point a very basic point becomes visible through the fog: If rising CO2 does not reliably correlate with rising temperature over time the theory that it does is wrong.
Perhaps the global rent seeking frenzy should have stuck with the global cooling crisis of the seventies as the global mean temperature of unadjusted data shows consistent negative deviation from the 30 year mean over the last 7 years. http://temperature.global/
IMO the important takeout from your report is that to the measuring the anthropogenic impact on global warming requires a number of assumptions. Our collective knowledge of the climate system is so limited that substituting equally valid assumptions can give a large number of alternative measures. This is the unrecognised underdetermination problem. P