One reason that climatologists could confidently determine That humans are responsible for modern warming lies because they have more than just weather data to work with. There are many places where a human cause can be identified if you know how to dust fingerprints. For example, as the lower atmosphere warms, the stratosphere actually cools. That's what you expect when greenhouse gases ̵
A new study by Ben Santer of the Lawrence Livermore National Laboratory searched for fingerprints in a new location: the seasonal temperature cycle. The ideal tool to analyze this is the global temperature record of satellites that began their observation in 1979. That is, they do not go nearly as far back as weather station rates, but the dataset is now long enough to be useful studies like this.
Hot and Cold
While all use the same satellites, several different groups actually maintain separate satellite temperature records. This is because the measurements are far from easy and a lot of work goes into all the processing necessary to spit out temperature maps. Therefore, the different sets of data do not always match perfectly – or with those analyzed with earlier versions of their processing algorithm. In this study, researchers therefore used the last two versions of three different datasets.
Each record tracks several different layers of the atmosphere. One record covers the lower troposphere – the first 10 kilometers above the Earth's surface. Although this comes closest to the surface temperatures we live in, it is also the most difficult measure to get it right. A cleaner record focuses on the middle troposphere, slightly higher up.
Detecting the difference between the coldest winter months and the warmest summer months – the extent of the annual cycle of seasons – some interesting regional patterns emerge. (You can see this in the picture at the top of the page.)
If you have averaged the northern hemisphere strip at mid-latitude, there is a greater seasonal temperature change than in the southern hemisphere, because there is a much larger area of land (the warmer and cooler to bigger extremes than the ocean). But this seasonal cycle has increased measurably since 1979, more so in the northern hemisphere, because the summer temperatures in the atmosphere are rising faster than the winter temperatures.
In the tropics, the seasonal cycle is essentially unchanged. In the vicinity of the poles, however, the satellites show that the seasonal temperature change has decreased.
There is an interesting exception that does not show this pattern: the University of Alabama in the Huntsville record operated by Roy Spencer and John Christy (two of the small handful of vocal contrarian scientists who reject or reject human-induced climate change downplay). This dataset looks completely different in the Antarctic. In fact, the craze in Huntsville's data only appears in its latest version update and seems to reflect a problem that addresses the transition between first and second generation satellites. There is a history of managers of other large satellite data sets discovering bugs in the Huntsville algorithm, and this seems to add another example.
Models and Reality
What do these altered seasonal patterns mean? To answer this question, Santer and the other researchers used a collection of important climate model simulations. These include long simulations of an unchanged (pre-industrial) revolutionary climate and simulations of human-induced warming until 2016. This allowed them to analyze the seasonal patterns that predict climate models for a warming world and the extent to which these seasonal patterns can naturally vary.
The researchers found that the models predict almost exactly this seasonal pattern – greater seasonal fluctuations in mid-latitudes (especially for the northern hemisphere), minor changes in the tropics, and minor fluctuations in the Antarctic. The models also forecast a smaller seasonal cycle around India and Southeast Asia that deviates from the general trend of mid-latitudes.
The one difference is in the Arctic, where the satellite data show a greater decline in the seasonal cycle than the average model predicted. About a third of the models show a decline, the rest not. The models have generally underestimated the loss of Arctic sea ice, and perhaps this is the key, as the loss of sea ice is an important factor in the seasonal cycle change.
It's We, Version 857
To Test How Strong The changes observed by the satellite point to man-made warming. The researchers used a signal-to-noise analysis. Using simulations prior to the Industrial Revolution to estimate the range of natural variability, the analysis showed that the observed seasonal patterns in the middle troposphere occurred well above the noise. This makes them a clear manifestation of the patterns predicted by the models. It is even more noticeable than the change in global average temperatures, because seasonal patterns across whole latitudes do not differ much in a stable climate.
While they were at it, researchers repeated these analyzes for a more direct measure of climate change: the annual average of temperatures analyzed at sites around the world. They write: "We find here that for the annual mean [mid-troposphere temperatures] the estimated [signal-to-noise] ratios exceed 4.4 for temperature changes over the 38-year satellite record, corresponding to a probability of 5 to 1 million, the annual ratios [signal-to-noise]]
Overall, they conclude: "The best explanation for these results is that the basic physics and fundamental physical mechanisms propel the large-scale changes in [seasonal patterns]." For the tropospheric temperature is one of man This signal is now recognizable in the seasonal cycle itself. "
That's just one more reason why scientists can be certain that humans are driving climate change forward.
Science 2018 DOI: 10.1126 / science.aas8806 (About DOIs).