You climate scientists and your models, part 2

My previous post was about the notion that models don’t constitute “evidence” for propositions in climate science. In this post I write about the criticism, also expressed to me in comments after my New York Times op-ed, that climate models (or the tools and methods of climate science more broadly) have been disproved by observations, so we should ignore them.

The people who wrote this to me didn’t spell out what disagreement they were talking about. But I assume that these comments were stimulated by the way I framed the op-ed piece, in that I wrote at the start about the major hurricane “drought” in the US over the last decade. The commenters seemed to be implying that since climate scientists had predicted stronger hurricanes in the past, the fact that the last ten years of Atlantic hurricane seasons have been relatively quiet disproves climate models, or climate science generally.

I set up the piece that way to be provocative, and to make a connection between current events in the US and the broader questions addressed in our Science paper that motivated the Times piece. But the point of the whole op-ed piece, as well as the Science paper, was that our recent history is, in fact, consistent with our current projections of long-term trends towards stronger hurricanes worldwide. My most vehement critics didn’t seem to seriously consider the arguments I made, as I didn’t get any responses which refuted them in any substantive way. But let’s elaborate a bit here on the two basic issues which our Science paper raises, one at a time, and then come back to the issue of whether the models, or the predictions made by climate scientists overall, are any good or not.

The first issue, very well known by the scientific community, is that there is large natural variability in hurricane activity. Hurricane seasons in different parts of the world are doing different things at different times, and they all fluctuate due to natural causes. So if you want to see climate change trends, you have to wait a while for them to become clear. Think about investing in the stock market for your retirement. Let’s say you start investing in your twenties and plan to retire at 65. Is the market going up or down over your working lifetime? You can’t tell by what happens from one day, one year, or even one decade to the next. That long term is what climate change predictions are about.

And you have to wait longer to see trends in hurricanes than you do to see trends in other variables. Take temperature for example. There are many more measurements of temperature than there are of hurricanes, since there aren’t many hurricanes while temperature can be measured anywhere. So statistical analyses of temperature data are much easier than those of hurricane data, because the sample is larger. Looking for temperature trends is like looking for stock market trends in a broad index like the S&P 500, made up of many stocks, while looking for hurricane trends is more like looking at an average of just a few stocks. The average of just the few will be more subject to the vagaries of what’s going on with those few companies day-to-day, and will have much more volatility, while the S&P 500 will show broader market trends more clearly. Looking at the hurricanes in just a single basin, like the Atlantic, reduces the sample size still further and makes the data even less representative of global trends. Continuing our stock market analogy, looking at the Atlantic only is like looking at just a single stock. It may be important, if you’re interested in that particular one, but it doesn’t necessarily tell you much about the bigger picture of the whole market.

Now, there are several credible studies by very established researchers that do, in fact, find that increasing trends in hurricane activity are already evident in recent historical data, either in individual basins or globally. But there is some disagreement between studies on the of statistical significance of these trends. So one argument is that the long-term trends are there, but that we need to wait longer to see them rise further above the noise so that everyone agrees they are there and significant.

Regardless, though, the notion that the hurricane drought in the Atlantic has somehow disproved the consensus projections of climate science is wrong, because the drought is still a relatively short-term fluctuation in a single basin, while the projections are for long-term global trends.

The second issue raised in our Science paper (now available free, see bottom of this post) is that perhaps there shouldn’t yet have been substantial long-term trends in hurricane intensity – whether we would be able detect them above the natural variability or not – because until the last couple of decades, aerosol cooling effects on hurricanes have been counteracting the effects of greenhouse gas warming. Though not a new idea either, this is one that’s perhaps a bit less understood than the one about natural variability. It has been raised before mostly with regard to the Atlantic, while to my knowledge our paper may be the first to show evidence from the IPCC climate models that aerosol effects have been important for hurricanes globally. It’s a subtle argument, because aerosol cooling has clearly been less than greenhouse warming – if not, the planet wouldn’t have gotten warmer over the last century. But it appears that aerosol effects on hurricane intensity are disproportionately stronger than their effects on the climate overall, because they reduce incoming sunlight while greenhouse gases act through longwave (infrared) terrestrial radiation, and changes in sunlight have a bigger influence on tropical cyclone intensity than changes in infrared radiation do.

To the extent the aerosol cooling estimates in the climate models are accurate, potential intensity theory (discussed in the previous post) implies that hurricane intensities shouldn’t have started to increase at all until the 1980s or 1990s, even though by that point the planet had warmed quite a bit. This would further delay time when the increases would be detectable above the noise of natural variability. The aerosol cooling in the models may be wrong to some extent, but is almost certainly in the ballpark enough so that the aerosols have compensated for the greenhouse gas effects on hurricane intensity to a significant degree, even if the exact degree is different than what’s shown in our paper.

So, to summarize: the projection that a world warmed by greenhouse gases – absent other compensating factors – will have stronger hurricanes still holds, but a) large natural variability means that it may take a while to see the trends, and the Atlantic “drought” is just part of that variability, and b) aerosol cooling can compensate the greenhouse effect on hurricane intensity, and has done so for much of the historical record until recently, slowing down the intensity increases. The role of natural variability in obscuring long-term trends has been understood (at least broadly) for a long time. The role of aerosols has emerged over the last decade and is still not fully appreciated.

Back to the broader issue of whether climate models have been disproved, though: the questions I’ve just written about are all about how climate change affects hurricanes, and not about the basic fact of human-induced climate change itself. Because hurricanes are so rare, and their natural variability so large, they are among the least clear indicators of climate change. There are plenty of much clearer ones, from increasing surface temperature (the global warming itself), to melting of ice on land and sea, to the long-term cooling of the stratosphere, increasing intensity of heavy rain events, etc.

When we consider all these climate change predictions as a whole, there is now a good track record. Climate scientists have been making predictions for several decades. The evidence shows that not only have these predictions come true, but they have tended to be conservative, understating the rapidity, extent and impacts of human-induced climate change. There are still many uncertainties, but those are reasons to be more concerned, not less, since they mean the reality could just turn out worse than we expect just as easily as better, and that’s what seems to have been happening so far.

PS: If you want to read our Science paper, it is now available for free – legally – via links on my academic publications page. (I can’t put the link directly to the paper here, as Science gives us this free link on the condition that it only can be used from a single referring page.) It’s currently at the top of the list of papers on that page, but in any case look for Sobel, Camargo, Hall, Lee, Tippett, and Wing, 2016: Human influence on tropical cyclone intensity. Science, 353, 242-246, DOI: 10.1126/science.aaf6574.