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Common Questions about Climate Change
Published in 1997 by the United Nations Environment Programme - World Meteorological Organization




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Are Human Activities Contributing to Climate Change?

A comprehensive assessment by the IPCC of the scientific evidence suggests that human activities are contributing to climate change, and that there has been a discernible human influence on global climate.

Climate changes caused by human activities, most importantly the burning of fossil fuels (coal, oil, and natural gas) and deforestation, are superimposed on, and to some extent masked by, natural climate fluctuations. Natural changes in climate result from interactions such as those between the atmosphere and ocean, referred to as internal factors, and from external causes, such as variations in the sun's energy output and in the amount of material injected into the upper atmosphere by explosive volcanic eruptions.

Studies that aim to identify human influences on climate attempt to separate a human-caused climate-change factor (the signal) from the background noise of natural climate variability. Such investigations usually consist of two parts: detection of an unusual change, and attribution of all or part of that change to a particular cause or causes.

The concepts of detection and attribution may be understood in terms of a simple medical analogy. Measurement of a body temperature of 40°C (104°F) detects the presence of some abnormal condition or symptom, but does not in itself give the cause of the symptom. To attribute the symptom to an underlying cause often requires additional and more complex tests, such as chemical analyses of blood and urine, or even x-rays and CAT scans. Early work on climate-change detection examined changes in the globally averaged surface temperature of the Earth over the last century. Most studies of this type concluded that the observed increase of roughly 0.5°C (about 1°F) was larger than would be expected as a result of natural climate variability alone. Observed globally averaged temperature changes have also been analyzed away from the Earth's surface. The observations used come from conventional weather observing instruments (radiosondes) and from satellites. As expected, because of the different factors affecting the variability of and persistence of temperatures at different altitudes, there are noticeable differences between short-term trends at the surface and those at higher altitudes. The record of temperatures away from the Earth's surface, which spans only the past 40 years compared with the much longer surface record, is too short for globally averaged values to provide any definitive information about the extent of human influences.

The further step of attributing some part of observed temperature changes to human influences makes use of climate models, which have been employed to estimate the climatic effects of a range of human-induced and natural factors. The human factors include recent changes in the atmospheric concentrations of both greenhouse gases and sulfate particles (called "aerosols"). The natural factors considered include solar variability, the effects of volcanic eruptions, and internal variability of the climate system resulting from interactions among its individual components.

The changes in globally averaged temperature that have occurred at the Earth's surface over the past century are similar in size and timing to those predicted by models that take into account the combined influences of human factors and solar variability. To probe the question of attribution requires the application of more powerful and complex methods, beyond the use of global averages alone. New studies have focused on comparing maps or patterns of temperature change in observations and in models. Pattern analysis is the climatological equivalent of the more comprehensive tests in the medical analogy mentioned previously, and makes it possible to achieve more definitive attribution of observed climate changes to a particular cause or causes.

The expected influence of human activities is thought to be much more complex than uniform warming over the entire surface of the Earth and over the whole seasonal cycle. Patterns of change over space and time therefore provide a more powerful analysis technique. The basic idea underlying pattern-based approaches is that different potential causes of climate change have different characteristic patterns of climate response or fingerprints. Attribution studies seek to obtain a fingerprint match between the patterns of climate change predicted by models and those actually observed.

Comparisons between observed patterns of temperature change and those predicted by models have now been made at the Earth's surface and in vertical sections through the atmosphere (Figure 3.1). Model predictions show increasing agreement with changes observed over the past 30-50 years. The closest agreement occurs when the combined effects of greenhouse gases and sulfate aerosol particles are considered. Statistical analyses have shown that these correspondences are highly unlikely to have occurred by chance.

Figure 3.1

Modelled and observed changes in atmospheric temperature, from close to the Earth's surface to the lower stratosphere. Model results are from two sets of experiments: with "present-day" levels of atmospheric CO2 (panel a), and with present-day CO2, sulfur emissions, and stratospheric ozone depletion (panel b). They are given as changes relative to a pre-industrial state of the atmosphere. Observed changes (panel c) are temperature trends over the period 1963 to 1988, as estimated from weather balloons. All results are for annually averaged data and are in units of°C (panels a, b) and°C/25 years (panel c). The patterns of change in panels b and c are similar.

The agreements between the patterns of change predicted by models and those actually observed are due to similarities at large spatial scales, such as contrasts between the temperature changes in the northern and southern hemispheres or between different levels of the atmosphere. It is at these large scales that we have most confidence in model performance. More importantly, many of the results of these studies agree with our physical understanding of the climate system, and do not depend solely on numerical models or statistical techniques.

There are still uncertainties in these detection and attribution studies. These are due primarily to our imperfect knowledge of the true climate-change signal due to human activities, to our incomplete understanding of the background noise of natural climatic variability against which this signal must be detected, and to inadequacies in the observational record. Such uncertainties make it difficult to determine the exact size of the human contribution to climate change. Nevertheless, the most recent assessment of the science suggests that human activities have led to a discernible influence on global climate and that these activities will have an increasing influence on future climate.

Go to "What Human Activities Contribute to Climate Change?"

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