2010-12-02 10:56:36The human fingerprint in the seasons
John Cook


In 1896, Svante Arrhenius mentioned that greenhouse warming should cause winters to warm faster than summers (Arrhenius 1896), citing an earlier prediction by John Tyndall (Tyndall 1865). During summer, a region receives more sunlight and warms. During winter, the region receives less sunlight and cools by radiating heat to space. Greenhouse gases stop some of this heat from escaping to space so an increased greenhouse effect slows down the winter cooling. Consequently, if greenhouse gases are causing global warming, we expect to see winters warming faster than summer.

A pair of studies (Braganza et al 2003, Braganza et al 2004) recognise that within the temperature record are a number of climate indices that can tell us more about what's causing temperature change than mere global temperature. The difference in trend between summer and winter, between land and ocean warming, between the equator and the poles - these all hold vital clues into what has caused climate change since the instrumental record began in the 1800s.

They found that winters have been warming faster than summers. What's interesting is how the seasons have changed over time. In the early 20th century, they find the warming is a combination of man-made and natural forcing (eg - from the sun) as well as some internal variability (eg - ocean cycles). In the latter 20th century, man-made forcing accounts for nearly all the observed temperature changes (Braganza et al 2004).

Figure 1: Smoothed temperature anomaly of northern hemisphere land temperature, from CRUTemp. Polar regions are excluded using a fixed data mask (Braganza et al 2004).

Note - Figure 1 was emailed to me by David Karoly, one of the co-authors of Braganza et al 2004. However, while digging into this subject, I tried plotting the winter vs summer trend myself. Robert Way kindly helped out by working out the winter and summer temperature anomalies and plotting them  (here's the Excel file for the curious climate tragic).


Figure 2: Yearly temperature anomaly for winter (light blue) and summer (light red) plus five year moving average for winter (thick blue) and summer (thick red).

Not only does the faster warming winter provide evidence for greenhouse warming (on top of many other lines of evidence for man-made global warming), it also provides evidence that the sun isn't the cause of recent global warming. If global warming was driven by the sun, we should see summer warming faster than winter. This is just one of the "solar fingerprints" that we would expect to see from solar warming, that we don't see. Interestingly, many of the solar fingerprints are quite different to the patterns expected from greenhouse warming

For example, greenhouse warming predicts nights should warm faster than days while solar warming is the other way around. Observations are consistent with greenhouse warming. Similarly, if global warming was driven by the sun, we should see the stratosphere warming as well as the troposphere. Greenhouse warming, on the other hand, warms the troposphere but cools the stratosphere. Again, observations match greenhouse warming.

Solar warming should result in the tropics warming faster than the poles. What we observe instead is the poles warming around 3 times faster than the equator. All these pieces of evidence paint a consistent picture - greenhouse gases, not the sun, are driving global warming over the past few decades.
2010-12-02 20:29:05

The link in fig.1 and the next are to Braganza 2003, not 2004.
The two figures say the same thing, the most recent (yours) is enough.
Should you decide to keep both, people will ask why in fig 1 winter temperatrure goes down around 2000 while in your graph it does not.
You should quote the dataset used and the geographical mask, if any.
In the legend what "per." stands for?
What is the baseline of the two curves in fig. 2?

2010-12-07 16:55:01Polar amplification
James Wight

How is polar amplification a greenhouse fingerprint? Shouldn't there be an ice albedo feedback regardless of whatever causes the warming?
2010-12-07 17:10:42Re: Polar Amplification
Daniel Bailey
Daniel Bailey

From BPL's Is it the sun? page:

"So why polar amplification? There are two reasons. One is "ice-albedo feedback"--as more bright polar ice melts, more dark land or seawater is revealed to the sun, and dark things absorb light better than bright things.

Another is that colder air holds less water vapor. Water vapor is the dominant greenhouse gas in Earth's atmosphere. In cold regions with less water vapor in the atmosphere, carbon dioxide accounts for proportionally more of the greenhouse heating. So with carbon dioxide rising, you get faster global warming the closer you get to the poles."

2010-12-07 19:11:05I still don’t understand
James Wight


But as I understand it, throughout the glacial/interglacial cycles there was more warming at the poles than the equator. That warming was due to Milankovich cycles. Wouldn’t Milankovich warming have the same pattern as solar warming?