Introduction

This unit provides an introduction to global warming. We will be considering the history of global warming by looking at the pattern of ice ages and analyisis of recorded temperatures. We will aim to gather meaningful information from this data. We will briefly assess the impact and influence of humans on global warming and, finally, we will examine climate models and how to predict future changes.

2. A 4.6 billion-year history

Climate change is a natural process of warming and cooling that has occurred all through the Earth's history. Throughout geological time there have been 'hot-house' periods and ice ages. In order to understand the current situation, it is necessary to have some sense of context and perspective, from historical and geological time-scales.
We are currently enjoying an interglacial period of an ice age that began approximately two million years ago. (Ice ages are composed of colder 'glacial' and warmer 'interglacial' periods.) This is at least the seventh ice age in the Earth's 4.6 billion-year history.

During the Cretaceous (65-147 Ma) the whole Earth was up to +15 (C warmer than at present, with tropical forests covering Antarctica, whereas during the Quaternary (∼2 Ma) ice sheets spread across much of Europe and the mean surface temperature was up to (5 (C colder than at present. The range of these changes is much greater than the observed increase in temperature over the past century (+0.6 ±0.2 (C) and predictions for the next hundred years (+3 ±1.5 (C).

3. Recorded temperatures

Analyses of over 400 proxy climate series (from trees, corals, ice cores and historical records) show that the 1990s was the warmest decade of the millennium and the 20th century the warmest century. The warmest year of the millennium was 1998, and the coldest was probably 1601. (Climatic Research Unit, 2003)

Throughout historical times, fluctuations in the Earth's mean temperature have been recorded. During the seventeenth century, the Thames periodically froze over during winter and mini-glaciers were present in the North West Highlands of Scotland. More recently, the 1990s included some of the hottest years ever recorded in the British Isles, and 10 August 2003 was the hottest day ever on record. An annual temperature record for central England has been constructed, beginning in 1659.

4. Human influence?

Evidence suggests that global temperature is beginning to rise. There are several factors that could cause this. Only one is affected by human activity. Up to this point we have been primarily concerned with evidence and trends, with 'what has happened'. This sort of analysis does not tell you 'why' something happened. To understand 'why', we need some sort of explanatory model that works on a global scale. The starting point of 'why' for climate change, is to consider energy flows in the climatic system.

5. Climate models
Climate models

To understand climate change it is necessary to construct climate models, to explore and predict interactions between different factors. Models are tested for accuracy against known sets of data, before being run forward to predict future changes.
It is clear from IPCC comparison charts between modelled and observed temperatures since the year 1860 that both natural influences and anthropogenic forcing are required to explain the recent rises in temperature.

5. Climate models
Running the models forward

What happens when the models are run forward? It depends upon the models used and the scenarios they are asked to run. It seems almost certain, however, that there will be increases in the global mean surface temperature, to the order of +1.5 to +4.5 (C - possibly more, according to some models and scenarios.

These changes are predicted to be associated with increases in sea level, changes to weather conditions (e.g. more regular and violent winter storms in the UK) and changes to the distribution of habitats and their associated organisms.

5. Climate models
Closing thoughts

Of course, doing anything about this needs scientific evidence and understanding, but it also requires social, economic and technological changes, which can only be achieved through political will. If you want to explore some of the broader context, a good place to start would be the New Internationalist issue 357, 'The Big Switch: Climate Change Solutions' at New Internationalist.
Faced with the sort of predictions climatologists are making, is it sufficient for science teachers to stop at the 'science', or should we be tackling some of these broader issues in schools?

If you want to find out more, the Hadley Centre is the UK's foremost climate research centre and provides a lot of useful information, as does the Intergovernmental Panel On Climate Change at IPCC.

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