Prioritize...
At the end of this section, you should be able to
- Describe why surface temperature is straightforward to measure and summarize key trends observed in global surface temperatures over the past century.
Read...
In the last section, we talked about the increase in greenhouse gases due to human activities over the last few hundred years. Combined with our knowledge of how these gases work from earlier in the class, our working hypothesis is that they lead to an increase in the temperature of the planet by increasing absorption of upwelling radiation and emission of downwelling longwave radiation to the surface. But is that really what we see when we look at observations?
Lucky for us, surface air temperature is relatively easy to measure and is a variable that is (almost) always reported anytime someone makes a weather observation. Instrumented surface air temperature measurements consisting of thermometer records from land-based stations, including islands, and ships provide us with more than a century of reasonably good global estimates of surface air temperature change. There is a particularly high-density network in places where people have lived, but even rural areas generally have samples from farmers and ranchers (and passionate amateur meteorologists!). Some regions, like the Arctic and Antarctic, and large parts of South America, Africa, and Eurasia – either due to a lack of people or means for temperature measurement – were not very well sampled in earlier decades, but records in these regions become available as people moved into them in the mid and late 20th century. Today, we now cover the planet with surface air temperature measurements using an expansive observer network.
Did You Know
Did you know that aside from penning the Declaration of Independence, Thomas Jefferson was an avid amateur meteorologist? His fascination with weather began years before the American Revolution, meticulously recording daily weather readings near his Virginia home. On his pivotal journey to Philadelphia for the Declaration's signing, Jefferson didn't forget his passion, stopping to buy a thermometer. He even devised a plan in the 1770s to establish a network of weather observations across Virginia, distributing thermometers for consistent data collection. You could argue that Jefferson was a bit before his time – his thoughts foreshadowed the National Weather Service's extensive network of over 12,000 weather stations nationwide. On the historic day of July 4, 1776, Jefferson recorded conditions in Philadelphia as mostly sunny in the morning, with increasing clouds and temperatures in the mid-70s by the afternoon—a fair-weather backdrop for America's birthday!
So, what do our observations of air temperatures tell us when it comes to looking at how they change over time? Remember way back to our early lectures — air temperature variations are typically measured in terms of anomalies relative to some base period. Using anomalies helps us see changes over time more clearly, especially in climate data. Since absolute temperatures can vary widely between locations, comparing against a standard base period (like 1951-1980) gives us a clearer view of how much each region has warmed or cooled over time.
The animation below is taken from the NASA Goddard Institute for Space Studies in New York City (which happens to sit just above “Tom's Diner” of Seinfeld fame), one of several scientific institutions that monitor global air temperature changes. It portrays how temperatures around the globe have changed in various regions since the late 19th century. The temperature data have been averaged into 5-year blocks, and reflect variations relative to a 1951-1980 base period. Red regions are warmer than the 1951-1980 average, whereas blue regions are colder than the 1951-1980 average, by the amounts shown. You may note a number that appears in the upper-right corner of the plot. That number indicates the average temperature anomaly over the entire globe at any given time, again, relative to the 1951-1980 average.
Video: Surface Temperature Patterns (1:19)
Let's look at the pattern of surface temperature changes over the past century.
We are looking at surface temperatures relative to a base period from 1951 to 1980. So we're looking at whether the temperatures are warmer or colder than the average temperature over that late 20th-century baseline. And we're looking at five-year chunks.
We can see that in the 1930s, for example, there was some warming at high latitudes but not global in nature. We can see that in later decades, the 1960s to 1970s, there was some cooling over large parts of the Northern Hemisphere... hold that in your brain. That might have had, in part, a component due to aerosol production by human activity.
And of course, as we get into the late 20th century, we see large-scale warming that is unprecedented over at least the period covered by the instrumental record.
Explore Further...
Take some time to explore the animation on your own. You may want to go through it several times, so you can start to get a sense of just how rich and complex the patterns of surface temperature variations are. Do you see periodic intervals of warming and cooling in the eastern equatorial Pacific? What might that be?