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Global Warming: How Fast?
Posted on August 20, 2019 |
Here’s the annual average temperature for the planet Earth, from 1880 to now, according to data from NASA:
[Technical note: NASA reports temperature anomaly: the difference between temperature and its value during a “baseline” period which they chose as 1951-1980. I’m more interested in how much we’ve warmed since these records began so I’ve used a different baseline period, 1880-1899, hence values show how much we’ve warmed since then. Also, 2019 isn’t over yet so I’ve shown the year-to-date value.]
Clearly, Earth got hotter. Just as clearly, it doesn’t just follow a smooth trend over time, it also fluctuates from year to year, substantially, apparently randomly. Just as clearly, the trend it has followed has not been a straight line. Sometimes it was warming slowly, or even cooling off (especially during the early years), other times it was warming rapidly (especially recently).
We can get a good statistical model of the trend by using more than just a single straight line, we can use pieces of straight line segments joined at the ends. The “optimal” such model (if we can call it that) is this one:
It’s made of four straight-line segments, with the changes from one to the other happening in 1911, 1941, and 1970. All the parameters — the line segments themselves as well as the moments at which they begin and end — were chosen to make the model fit the data best.
According to this model — and it’s just a model (a statistical model, not a computer simulation) — the solid blue line shows where the trend most likely is, and the light-blue area around it shows the range within which the trend might be. The warming/cooling rate has gone through four “episodes.”
During the first (from 1880 to 1911) the rate was negative, Earth was cooling. Then from 1911 to 1941 we warmed significantly. From 1941 to 1970, we cooled off but not by much; in fact the uncertainty in the warming rate is bigger than the warming rate itself, so we can’t really be sure it was cooling off, it may have been just levelling off. From 1970 onward, we’ve warmed rapidly.
That’s all we can really say with confidence (statistically speaking) about how fast Earth has warmed/cooled since 1880. That doesn’t mean it really follows this “four straight-line segments” model — in fact it probably doesn’t, at least not exactly — but this is all we can say about changes in the warming rate with confidence.
What we can say with confidence is that this model captures the averagewarming rate during each episode. The warming rate may not be constant, but the straight-lines model gives its average (and the uncertainty in the average) for each episode, with fidelity. And here is a graph of the rates themselves, i.e. how fast Earth was warming/cooling during each episode (again, solid blue showing the most likely values, light blue outlining the uncertainty range).
This clarifies how the average rate changes over time. First it’s negative (cooling), then positive (warming), then possibly negative but possibly zero (cooling/levelling off), then rapid long-lasting positive (recent warming).
The rate during the most recent interval (from 1970 to the present) is estimated at 1.83 +/- 0.13 deg.C/century. And that is one estimate of the rate of warming right now: its average rate since 1970.
We can use more complex statistical models, which allow for more changes than just the straight-lines model. We can compute a fancy smoothed estimate, and for that I like to use the LOWESS smooth. There are many choices to make, but my choices lead to this model (again, the solid line is the best estimate of the trend value, the light red area shows its uncertainty range):
This model too enables us to estimate the warming rate — but now it’s not just four episodes of contant rate. The rate is changing all the time, and the model suggests this (best estimate in solid red, uncertainty range in light red):
The central estimate right now is 2.17 +/- 1.01 deg.C/century, over 20% higher than the rate suggested by the straight-lines model. We can even compare the two models directly (smoothed estimate in red, straight-line model in blue):
So what can we say about Earth’s warming rate right now? Based on the smoothed model, I’d say it’s almost surely between 1.66 and 2.67 deg.C/century. The average rate from 1970 to now was between 1.70 and 1.95 deg.C/century. Note that the values from the straight-line model are well within the uncertainty range of the smoothed model, so we don’t have statistically significant evidence of any recent change in the warming rate since 1970. It’s certainly possible, we just don’t have solid evidence for it.
My opinion: the present warming rate is about 2.23 deg.C/century, but could be as low as 1.66 or 2.67. It might be substantially higher than its average-since-1970 value of 1.83, but then again it might not.
My other opinion: given that the uncertainty is so large, we should at least be prepared for the possibility that right now, Earth is warming substantially faster than it has for the last 50 years. If that’s true, it’s a matter of serious concern.
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