How Do We Know That The Record Warmth Of Recent Decades Is Not Just Some Naturally Occurring Fluctuation In The Earth's Temperature?

Whether natural variability could be the reason for the existence of a ‘significant’ global warming trend – contributing to rising levels of greenhouse gases in the atmosphere is highly contentious. These views are pushed by those who still remain sceptical about the link between climate change and human activity.

Temperature changes over the past 400 000 years reconstructed from the Vostok ice core, the longest continuous ice-core record to date. Introduction to Climate Change, UNEP/GRID, Arendal, Norway; Figure 3.5 Kunsthistorisches, Museum, Vienna/ Bridgeman Art Library;

This record tells us, for example, that the Earth entered into the most recent comparatively cold period of its history (known as the Pleistocene Ice Age) around 2.6 million years ago. On a geological time-scale, these Ice Ages are relatively rare, covering only 2–3% of the history of our planet. The characteristic feature of the current one (and there is no reason to suppose that it is finished) is evident in the above graph. Drilled in Antarctica, the Vostok ice core provides a temperature record that goes back several hundreds of thousands of years. Beyond about 10 000 years ago, it tells a story of an unstable climate oscillating between short warm interglacial periods and longer cold glacial periods about every 100 000 years – with global temperatures varying by as much as 5 to 8 °C – interspersed by many more short-term fluctuations
By contrast, global temperatures over the last 10 000 years or so seem to have been much less variable, fluctuating by little more than one or two degrees. In short, the interglacial period in which we live, known as the Holocene, appears (on available evidence) to have provided the longest period of relatively stable global climate for at least 400 000 years. It is almost certainly no coincidence that this is also when many human societies developed agriculture and when the beginnings of modern civilisations occurred. We now shift the focus to the more recent past – the period during which human population growth and the coming of the industrial age began to make their mark on the composition of the atmosphere.

Contested science: a case study
For complex issues such as global climate change, there are many opportunities for scientists to take issue with the findings of their colleagues. They can disagree about the procedures for gathering data, the completeness or coverage of the data, how the data are analysed and interpreted, and then finally the conclusions. The assumptions that shape a particular piece of research and inform the kind of questions that will be asked can be no less contentious than the quality of the data gathered.

Such contention is not unique to climate science, of course. Fuelled in part by very human concerns such as a desire to protect one's reputation, competition for funding, etc., vigorous debate is the lifeblood of science; it helps to drive further investigation and innovation. In scientific areas where society has pressing concerns, however, influences beyond the normal cut and thrust of scientific debate come into play. Scientists are typically aware of the potential policy implications of their research, and may shape their work accordingly. Often, such research is stimulated or funded by organisations with an interest in the outcome of the policy debate. In turn, interest groups and policy makers tend to adopt a ‘pick n' mix’ approach to the available scientific evidence, promoting research that reinforces their existing arguments and beliefs, and neglecting or criticising more uncomfortable findings. Equally, the influence of individual scientists sometimes owes more to their access to decision makers or the media than to the reliability of their knowledge.

‘Global Warming’. An OpenLearn chunk used/reworked by permission of The Open University copyright © (2007).’