The Earth's temperature
The chart below shows a record of the global mean surface temperature of the Earth compiled for the past 140 years. Clearly there is an upward trend, but what does a chart like this really show?
Figure 1 Chart of variations in the Earth's surface temperature
- How can we know the temperature of the planet over time-scales of billions of years?
- Is this present warming part of the Earth's natural temperature variation?
- What factors affect the global temperature and force it to change, and to what extent are these being affected by human activity?
- What are the best predictions for change over the next 100 years?
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 experiencing 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 period (65–147 million years ago) the whole Earth was up to 15 degrees centigrade warmer than at present, with tropical forests covering Antarctica, whereas during the Quaternary period (about two million years ago) ice sheets spread across much of Europe and the mean surface temperature was up to five degrees centigrade colder than at present. The range of these changes is much greater than the observed increase in temperature over the past century (an increase of within 0.2 degrees of 0.6 degrees centigrade, ) and predictions for the next hundred years (an increase of between 1.5 and 4.5 degrees centigrade).
Taking it further
This article is an extract from an Open University OpenLearn course unit. Try a free course extract on global warming.
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