Now that global temperature has skyrocketed, talk about a “pause” of global warming only embarrasses the deniers who so craved it. Hence they’ve switched — again. Their general strategy is to search long enough and hard enough to find one thing that looks like it’s a sign against warming, which is easy even in a warming world; random fluctuation alone ensures there’s always at least one thing that’s bucking the trend. Temporarily, that is; because global warming is real, those fluctuation-induced signs don’t last. When the one deniers have been crowing about (like the so-called “pause”) turns out not to be, they switch to a different talking point.
Lately they want to focus on computer models, in the vain hope that by discrediting them they can discredit all of climate science. To hear them talk, computer models which simulate climate are completely wrong about everything, especially temperature, without a hope of a clue of how fast Earth is heating up, or will heat up.
The results of computer simulations are shared via CMIP (the Coupled Model Intercomparison Project), and global average surface temperature from the latest versions (CMIP5) can be downloaded here. There are lots of model runs included, some of which only simulate what we’ve already seen historically. To simulate what hasn’t happened yet, we’d need to know what future greenhouse-gas emissions will be like (and other things too, but mainly greenhouse gases). These are different for different computer simulations, but generally follow one of several possible “representative climate pathways” to cover a range of possibilities — we might get our act together and reduce emissions quickly, or we might keep burning fossil fuels rapidly in a “business as usual” way. The main representative pathways used for model runs are, from least to most carbon emissions, RCP26, RCP60, and RCP85.
Let’s compare the model results for RCP85, the high-emissions scenario, to observed temperature data from NASA. One of the important factors is that the models return surface air temperature, or SAT, while the observed data are a combination of air temperature over land and sea-surface temperature. This means that observed data underestimate the changes to SAT. However, NASA, in addition to publishing a land+ocean temperature index, also publishes an index based on meteorological stations only. So, we’ll compare model resuts for SAT to NASA’s data for the land-ocean temperature index (“loti”) and for meteorological stations (“met”), with the expectation that true SAT is somewhere between NASA’s two indexes.
I’ll limit the model results to the time span from 1880 to 2020, since observed data from NASA only go from 1880 through April 2016. There’s also the issue of how to align the data, i.e. what baseline to use for computing anomalies. This is something that can be abused to make comparisons misleading, especially if the baseline period is so brief that the records aren’t properly lined up. I’ll use the entire 20th century, which I define as January 1900 through December 1999, as baseline for both NASA data and for model results.
I managed to acquire monthly global temperature for 93 model runs for the RCP85 scenario. I’ve plotted them here in gray, together with the NASA monthly loti in red:
That alone shows that the models do a pretty good job estimating temperature change, including recently (when they’re forecasts rather than hindcasts). We can also see this, with a bit more clarity, by plotting yearly rather than monthly averages (but be advised, the final average has only 4 months’ data, not a full year):
There’s still the issue that NASA’s loti underestimates the increase of SAT. A similar comparison of model results with NASA’s met-station index (which overestimates the increase of SAT) shows this:
Annual averages look like this:
Clearly, contrary to what the deniers want you to believe, the models taken as a group have not overestimated global warming. Yet for some reason, this patently false claim is one of their loudest and most frequent.
We can also compare recent trends from the models to those from the observed data. Here are trends since 1970 (with 2-sigma error bars) for all 93 model runs, compared to the trends for NASA data shown as horizontal lines (loti in blue, met-stations in red, 2-sigma error ranges as dashed lines):
It’s no surprise that not all models give the same results. Some warm faster; the cluster of 6 in a row with high trend rates since 1970 are all runs of the CanESM2 model, and there’s one high rate from GFDL.CM3. Some warm more slowly, especially inmcm4 and MRI.CGCM3.
For more recent trends, we can compute rates since the year 2000:
Again, there’s plenty of variation among the models. Yet again, there’s no evidence at all that, taken as a group, they’re contradicted by observed temperature increase.
When deniers claim that models are useless and wrong, they’re not telling the truth. But there’s not much they can talk about these days. Surface temperature is through the roof, satellite temperatures are through the roof, ocean heat is through the roof, wildfires are burning down the roof, sea level is lapping at your ankles and still rising, Arctic sea ice is crashing through the floor, glaciers are vanishing before our very eyes, Greenland is melting at an alarming rate … When it comes to what’s happening with Earth’s climate, deniers have nothing to talk about that won’t embarrass their “don’t worry” narrative. Things have gotten so bad even Ted Cruz has shut up.
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