In the last post we saw that the planet earth, globally averaged, has been heating up. But a very important thing to know, something a lot of people aren’t aware of, is that not all places are warming at the same rate.
NASA provides very convenient maps of how temperature has changed over time, for different areas of the world. Here’s one, comparing the average temperature since 2010 to what the average was during the 1951-1980 period:
Different colors indicate different temperature differences since “back then” (the 1951-1980 time span), with blueish colors showing where it has cooled down, yellow-orange-red where it has warmed up. Areas colored white are those where the change has been between -0.2 and +0.2 °C, and may have gotten a little bit hotter or a little bit colder. Note that gray areas mark those places where we don’t have enough data to really know with confidence.
Since this is comparing an 8-year period to a previous 30-year period, most of the effect of short-term fluctuations has been eliminated; what remains shows us mainly the changes the trend has brought about.
It’s obvious that different places have warmed differently. In fact, some places haven’t really warmed at all! Note, for instance, that there’s a big “blob” of white-colored ocean in the Atlantic, just south of Greenland. There’s even more area that hasn’t warmed (much, if at all) in the southern hemisphere, mostly far to the south, around the Antarctic continent. There are even places near the Antarctic coast colored light blue — indicating those regions have cooled since the 1951-1980 “baseline period” between -0.5 and -0.2 °C.
But there’s much more territory, in fact most of the earth, that has warmed. This is especially true of the northern hemisphere, while overall the southern hemisphere shows considerably less warming.
It’s also clear that land areas have warmed more than ocean areas. In large part this is because of thermal inertia. It takes energy to heat something, and the oceans require a lot more energy to warm by a given amount than land areas. That’s one of the reasons the northern hemisphere has warmed more than the southern; most of the world’s land is in the northern half of the globe, while the southern half is much more dominated by ocean.
The most extreme warming is in the very far north, in the Arctic. The globe as a whole has warmed by about 0.75°C since the 1951-1980 baseline period, but Arctic temperatures have increase much more, between 2 and 4°C. This has brought about melting of glaciers and ice sheets in the far north, which adds more water to the ocean and therefore contributes to sea level rise.
NASA doesn’t just provide estimates of global average temperature, they also estimate average temperature for different latitude zones of the earth. Let’s take a look at some of those data, and see firsthand how different latitude zones have changed temperature over time.
We’ll look at three large latitude areas. The southernmost will extend from latitude 24°S down to the south pole; we’ll call that the “South Extratropics.” The northernmost will extend from latitude 24°N up to the north pole, we’ll call it the “North Extratropics.” The middle will cover the area from latitude 24°S to 24°N, and we’ll call it the “Tropics.” Here are yearly average temperature anomalies for those three regions — the south in blue, the north in red, the tropics in brown:
All three of these large regions have warmed since 1880, but the South Extratropics has warmed least, the North Extratropics most.
This is clearer if we remove most of the year-to-year fluctuation by fitting a smoothing function to the data sets. That will reveal what the long-term trend is doing better than the yearly data itself. And it looks like this:
Now it’s obvious that the North Extratropics has warmed a lot faster than the Equatorial region while the South Extratropics has warmed a bit more slowly.
Let’s look at another, smaller latitude zone, from latitude 64°N to the north pole, which we’ll call the “Arctic.” Here’s the temperature for that zone from NASA:
I’ve also included a smooth-fit curve as a thick red line. Consider the question: how much has the climate warmed in the Arctic since 1880?
One might be tempted to take the difference between the lowest and highest values, or the difference between the year-1880 value and the year-2017 value. But those include much of the year-to-year fluctuation in temperature, which is part of the noise in the data. We really want to know how much the climate has warmed, not how much the noise fluctuations around. Therefore a better idea is to find out how much the smooth curve has warmed, because it’s a better indicator of climate changes without the confounding influence of the fluctuation noise.
If we did that with global average temperature (for the whole earth) we’d find that the planet has warmed by about 1.1°C since 1880 (about 2°F). But the Arctic has warmed much more, by around 3.6°C. That’s more than three times as much. Also, we can estimate how fast the climate is warming right now; for the globe as a whole it’s about 0.018°C/year, but for the Arctic it’s more like 0.072°C/year. That’s four times as fast as the globe.
I often see, in news reports and on blogs, that the Arctic is warming twice as fast as the global average. Be aware: that’s an underestimate. The Arctic is warming at least three times, maybe even four times as fast as the globe.
We said earlier that land areas are warming faster than ocean areas, hence faster than the globe as a whole. How much faster? Here’s earth’s average temperature over land areas only, since 1880:
Once again I’ve included a smooth-fit curve, according to which earth has warmed by 1.44°C since 1880, about 30% more than the global average. The present rate of warming over land areas is about 0.028 °C/year, more than 50% faster than the warming rate for the globe as a whole.
The land-area warming is quite important, because that’s where we live.