2 What do we know about recent climate change?

2.1 Preamble

Here are some quotes from the ‘Summary for Policymakers’ (SPM) included in the report from the scientific working group in the IPCC TAR (IPCC, 2001a):

• The Earth's climate system has demonstrably changed on both global and regional scales since the pre-industrial era, with some of these changes attributable to human activities.
• Globally, it is very likely that the 1990s was the warmest decade and 1998 the warmest year in the instrumental record [1861–2000].
• New analyses of proxy data for the Northern Hemisphere indicate that the increase in temperature in the 20th century is likely to have been the largest of any century during the past 1000 years. It is also likely that […] the 1990s was the warmest decade and 1998 the warmest year [of the millennium].
• In the light of new evidence and taking into account the remaining uncertainties, most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations.

The overall aim of this section is to review the scientific evidence supporting these conclusions. For example, how sure are scientists that the Earth really is warming up? Specifically, what do terms such as ‘very likely’ and ‘likely’ actually mean? And how do we know that the record warmth of recent decades is not just some naturally occurring fluctuation in the Earth's temperature that has little, if anything, to do with human activities?

As you will see, the ‘background noise’ of natural variability makes establishing the existence of a ‘significant’ global warming trend – one that could be due to rising levels of greenhouse gases in the atmosphere – not only difficult, but also highly contentious. Work that challenges the mainstream view on this issue, embodied in the IPCC consensus, is commonly cited by those who remain sceptical about the link between climate change and human activity. We look at one recent example, set in the political context of the day, later on in the section. First, we focus on what is known about variations in the Earth's temperature over a range of past time-scales.