Committee on Climate Change

We see unequivocal evidence that the world is warming, not just from commonly-cited measurements of global average surface temperature, but also from a range of indicators (see Figure 1.5 below) measured by a variety of research groups around the world:

• Near-surface air temperatures over land are increasing.
• Near-surface air temperatures over the oceans are increasing.
• Sea surface temperatures are increasing.
• Ocean heat content is increasing.
• Sea level is rising (water expands as it warms, and there are additional contributions to sea level through melting of ice and snow on land).
• Atmospheric humidity is increasing (warmer air is able to hold more moisture).
• The temperature of the lower atmosphere (troposphere) is increasing.
• Northern hemisphere snow cover during March-April is decreasing.
• Total glacier mass is decreasing.
• Arctic sea ice extent in September is decreasing (Arctic sea ice changes cyclically over the year, usually reaching a minimum in September).

 
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The last decade has seen the warmest global average surface temperatures since records began, about 0.8°C above pre-industrial levels. This is despite 1998 frequently being cited as the warmest year, Although there is variability between years and regions, the long-term trend still shows warming:

• Data from three separate research groups, all measuring global surface temperature averaged over both land and sea, agree that 2000-2009 was the warmest decade on record

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• Small differences in the calculation methods mean that 1998 appears as the hottest year in one record (HadCRUT, produced by the Met Office and the Climatic Research Unit), whereas 2005 appears hotter in the other two.
• It is known that natural variability can offset (or enhance) the surface warming trend from GHG emissions for periods of up to a decade or so. Scientists therefore tend to use 20- 30 year periods in order to identify climate trends over the noise of short-term variability. From this perspective it is clear that temperatures are on a long-term rising trend.

It is close to certain that the planet has warmed since the late 19th Century.

How do we know this is due to human activity and not other, natural factors?

The combination of climate observations, measured GHG emissions and a fundamental scientific understanding of the greenhouse effect provides a strong chain of logic linking human activities to current global warming.

While natural variability and other factors continue to play a role in climate, the pattern of warming also suggests they are not the primary drivers of change in the last few decades:

• Natural variability within the climate system is unlikely to explain such a large worldwide increase, sustained for so long. Although we cannot rule out some form of large internal variation, as yet poorly understood, studies of climate in earlier centuries suggest that the current trend is unusual in the context of natural variation. Furthermore, if the surface was being warmed by heat transfer from another part of the climate system, such as the oceans or the cryosphere, then we would see a cooling signal in those parts. In fact, however, warming is seen in both the oceans and in the melting of global snow and ice, consistent with the influence of an external source such as the Sun or GHGs (see charts above).
• Total Solar output shows a clear 11-year cycle of about ±0.08% since the late 1970s, which is when direct measurement from satellites became available. There is still more to learn about the influence of solar variability on climate. However, there is no clear increasing trend over recent decades. Furthermore, increased output would warm all parts of the atmosphere, including the lowest few kilometres (the troposphere) and the layer above (the stratosphere). Results suggest that the stratosphere is in fact cooling while the troposphere warms. This is consistent with forcing from GHGs instead (see Figure 1.9 below).

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There is a high degree of confidence that human emissions have caused most of the observed warming since the mid-20th century.