Climate Change: Patterns Of Evidence

  • Date: 08/11/13
  • Dr David Whitehouse

It has been said often that man-made climate change is not revealed in any specific climate parameter but by a “pattern of evidence.” So what is happening to some of the potential candidates for inclusion in the climate change “pattern of evidence?”

Any one changing parameter, such as global temperature, ice sheet changes, stratospheric cooling, changing seasons etc., is not taken as particularly significant in itself. It is when they all change together in a consistent direction that their significance exceeds the sum of its parts, and the case for strong anthropogenic climate change is made.

It’s an approach that has been used for many topics such as the rationale for the search for intelligent life in space and the justification for the International Space Station. But the “pattern of evidence” argument has always struck me as something to be particularly careful with. How do you define evidence? What evidence is included and what excluded? How many factors should be considered, and over what timescale? What do you do about ‘inconvenient’ factors that go against the perceived trend? It seems to me there is a lot of scope for bias and selection effects if not cherry picking of just the ‘evidence’ that supports a particular point of view.

So what is happening to some of the potential candidates for inclusion in the climate change “pattern of evidence?”

Temperature change

It’s common knowledge that the global surface temperature (land and sea) show no change in the last 16 or 17 years. The climate models struggle with this.

Ocean Depths

Some are not worried by the surface temperature hiatus as they say that 90% of the excess energy from greenhouse gas forcing goes into the sub-surface ocean and that the data shows the temperature of the ocean’s intermediate and deep layers are increasing. The reality is however not as straightforward. The data are still unclear given the differences between the timescales and calibrations of the various systems employed to measure ocean temperatures. Work published in last week’s science journal suggested that the Ocean Heat Content was much higher a thousand years ago (when the surface temperature was comparable) and that it is currently increasing after a very low period during what is called the Little Ice Age in the 17th century.

Sea level

In contrast to what some scientists and activists say there is a strong body of opinion, backed up by peer-reviewed publications, that indicates sea level has been rising at a constant rate for a century, showing no recent acceleration, and no anthropogenic influence.

Arctic ice

Portrayed by some as the talisman of global climate concern Arctic ice has been declining steadily since satellite observations began in 1979. That the decline was already in progress should be taken into account when anthropogenic influences are considered. There might be an increase in the rate of decline in the last decade. Last year was a record low although brought about by an unusually severe storm in August. This year has seen a dramatic increase that will probably assist another increase next year. Monotonic decline or cyclic behavior? We need more data. Click on image to enlarge.

Arctic Ice Nov

Antarctic Ice

Increasing. Global sea ice extent is also not far from record levels since satellite data began in the 1970s. Click on image to enlarge.

Antarctic Ice Nov

Stratospheric Cooling

It’s not simple but as the troposphere warms due to greenhouse gas forcing the stratosphere should cool. The data suggest that it is but not by as much as most computer models predict. There is also the cooling influence caused by the depletion of ozone. The coolest stratospheric year was 1996.

Extreme weather

Some argue that as there is now more energy in the climate system our weather will become more variable and intense. The problem with this point of view is that there is no evidence for it. As the IPCC SREX report indicated a few years ago there is no trend in extreme weather events apart from warmer nights and a technical definition of extreme precipitation. Some scientists have performed statistical analyses of heatwaves and concluded that whilst they are within natural variation they have occurred more often due to climate change. These studies are very speculative and preliminary and should be treated with caution.

Even at times when the global climate is relatively constant climate parameters will always be changing in various ways. Given the points made above with consideration of natural variability and uncertainties, one can ask if there is a pattern of evidence emerging?


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