In recent posts I’ve featured graphs of the average daily high temperature in Australia during summer. Here’s one of temperature throughout the entire year; black dots show each year’s average, while the red line with pink shading around it shows an estimate of the trend and its uncertainty range (just an estimate, mind you, but it’s based on math, not on “looks like”)
The trend line emphasizes two things Australians need to know:
Those fluctuations mean that even when climate doesn’t change, temperature will. There will still be hot years and cold years, but in a stable climate those fluctuations are limited. They can go high enough to make a hot year, but before the 2000s they could not reach high enough to make a year as hot as Australia just had. That only happened because a strong fluctuation — natural variability, some would call it — combined with an increasing trend.
And that’s one of the greatest dangers of climate change. Of course natural fluctuations can bring all kinds of trouble: heat waves, bushfire, flood, drought, storm — but when already-dangerous natural fluctuations combine with an increasing trend, that’s when disaster visits so often and wreaks such havoc that we wonder how we can cope.
Like … in Australia right now.
Some people want you to think it’s only due to “natural variability,” and that “global warming theory” simply can’t “explain” the temperatures in 2019. Like Roy Spencer, who, it seems to me, is both confused and deceived. My opinion: he was deceived into not just swallowing, but regurgitating the lies about arson being a reason the bushfire crisis is so severe. It’s not, that’s a despicable lie promoted by despicable people. Also my opinion: Roy Spencer is not one of those despicable people, and when it dawns on him how irresponsible it was to promote such a falsehood without bothering to find out the truth about it, he just might feel a pang of guilt. Just my opinion.
But what I found really interesting is that he claims “Australian average temperatures in 2019 were well above what global warming theory can explain, illustrating the importance of natural year-to-year variability in weather patterns (e.g. drought and excessively high temperatures).” His basis for “can’t explain” is that Australia has been even hotter lately than computer models of global climate were forecasting.
Let me paraphrase that for you: your lung cancer is even worse than the computer model forecast based on the cigarette/lung cancer theory, so it has nothing to do with your three-packs-a-day smoking habit. Yes, the analogy is “apt.” He supports his claim with this graph:
It shows observed temperatures (in red) compared to the average high temperatures over Australia simulated by 41 computer models (in blue). Here’s my own version:
I don’t know what 41 computer models Roy Spencer used, but I’ve done two versions, one for the “RCP2.6 emissions scenario,” the other for the “RCP8.5 emissions scenario.” Different models give different results, which is why what’s shown is the “multi-model mean,” the average of those that were used in the most recent IPCC assessment report.
You may have noticed that the computer model simulations don’t show fluctuations like the real world does; they fluctuate a bit, but just barely compared to observations. Actual computer simulations do show such fluctuations, but when you average together scores of different model runs, the fluctuations mostly cancel each other out, leaving just a little bit of residual fluctuation on top of the average trend from the models. The result is that what this graph really compares is the trend-plus-fluctuations from observations to the trend from the computer models.
“Global warming theory” doesn’t try to explain the fluctuations; they come and go with or without “global warming.” When Roy Spencer says “illustrating the importance of natural year-to-year variability” he’s trying to misdirect your attention away from the fact that it’s exactly the combination of year-to-year variability with relentless trend that makes disasters like Australia’s seem unbearable.
Climate deniers exploit fluctuations all the time. One way, that they’re using now, is to blame a crisis on “natural variability.” Of course natural variability is involved, but it’s a lie to say that means climate change isn’t involved. The other way, which we’re sure to see soon, is when fluctuations go in the opposite direction to the trend. Australia will probably have a cooler year or two before very long — fluctuations will fluctuate — and climate deniers will seize upon the opportunity to shout that Australia is in a cooling pattern and the “global warming theory” is wrong. I’ll talk about the trend. I’ll talk about the future.
The computer model simulations are imperfect, nobody claims otherwise, but they’re the best tool we’ve got to forecast what will happen, depending on what we do in the future. So far they’re doing a very good job projecting global temperature trends, but regional trends are far trickier and the models don’t do as well. They get the overall trend about right, but not with nearly as much precision as for the entire globe.
We can see that in the graph of Australia’s high temperatures comparing observations (trend-plus-fluctuations) to model results (model average trend). The observed trend itself shows signs of rising faster than the models project, not just fluctuating above, although the details of Australia’s present trend are still emerging. Nonetheless, the overall trend — its direction and timing, and approximately its rate — are in agreement.
Perhaps you’re wondering why I showed two different multi-model means. They’re for two different “emissions scenarios,” two different pictures of what we might do in the future. RCP2.6 is a low-emissions scenario in which we de-carbonize our economy and stabilize CO2 (and other greenhouse gases) rapidly. RCP8.5 is “business as usual,” meaning we keep burning fossil fuels like there’s no tomorrow.
The two scenarios are nearly identical in the graph above, because the two scenarios are nearly identical until very recently, they mainly differ in the decades to come. As a result, the model forecasts diverge widely in the decades to come:
The forecast in blue is what Australia might get if we get to zero-emissions soon enough. Temperatures will still be hotter than they were before, but maybe not much worse than they are already.
The forecast in red is what Australia might get if we keep doing what we’re doing. This is the Scott Morrison lump-of-coal future. Australians know how bad it is already. If the mean temperature rises that much more, how bad will it be when rising trend conspires with of those big fluctuations?
Australia: choose your future.
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