Climate change

The term climate change is used to refer to changes in the Earth's climate. Generally, this is taken to mean changes in the temperature, though 'climate' encompasses many other variables (precipitation, clouds, etc). 'Climate change' includes natural and anthropogenic forcing; 'global warming' is usually used to mean changes with predominantly anthropogenic forcing. For information on climate change over various periods, and the data sources available, see historical temperature record.

Table of contents

1 Climate change factors 1.1 Internal factors 1.2 Natural Factors 1.3 Human Factors 1.4 Radiative Forcing 1.5 Solar Radiation Variability 1.6 Examples of change due to natural factors 1.7 Anthropogenic greenhouse gases 1.8 Carbon Sources and Sinks 2 Evaluation of the relative importance of various factors 3 Attribution of climate change 4 General Agreement 5 Areas of disagreement 6 Global warming episodes in the geological record 6.9 Permian-Triassic extinction event 6.10 Paleocene-Eocene Thermal Maximum 7 See also

Climate change factors

Climate changes due to internal factors and external factors. Internal factors are those due to interactions within the earth's climate system. External factors are divided into natural factors, such as variations in solar radiation, and anthropogenic factors (those attributed to human activities).

Internal factors

It is known that the weather is a chaotic non-linear dynamical system. It is not clear that the climate (the average of weather) is such a system. Restricting ourselves to the last 400 kyr, the ice core record shows that the largest swings in climate are periodic, with the same periodicity as various orbital variations. These are thus non-chaotic. However, there are large short-term changes which do seem to be best explained as chaotic. Those variations do not seem to occur in the current climate state. Thus, it is possible that the climate system varies between chaotic and non-chaotic, depending on the state of the external forcing.

Natural Factors

It is clear that natural external factors have caused significant climate changes in the past, and it is probable that internal factors have too.

Human Factors

Anthropogenic factors are acts by humans (Homo sapiens) that change the environment and influence the climate. The major factor is CO₂ emission from fossil fuel combustion IPCC. Other factors include forest alterations, and agricultural or other changes that affect the Earth's albedo or the carbon cycle.

Radiative Forcing

The influence of external factors can be compared using the concept of radiative forcing. A positive radiative forcing warms the planet, and negative radiative forcing cools the planet.

Solar Radiation Variability

The main natural external factor is the variability in the amount, and geographic and temporal distribution of, solar radiation that reaches Earth. The solar radiation can change on short (yearly to century) timescales because of solar cycles and on century to millennial timescales because of cyclic changes in Earth's orbit. On much longer (hundreds of millions of years) timescales, the Sun is getting hotter.

Examples of change due to natural factors

The 100,000 year ice age cycle is due to natural causes. Within the last 1000 years, there are two extensive periods where temperatures were relatively warmer (the Medieval Warm Period) or cooler (the Little Ice Age). Since anthropogenic forcing is believed to be small then, it is assumed that these changes were due to natural factors. The Little Ice Age is usually attributed to the reduction of solar activity or increase in volcanoes; the causes of the MWP are even less clear.

A few scientists have claimed that the observed warming since 1860 is a natural climate recovery from the Little Ice Age. (Source: The Skeptical Environmentalist).

Anthropogenic greenhouse gases

The main anthropogenic factors are greenhouse gases, whose increased emissions add to the greenhouse effect; changes in land use; and the emission of aerosols such as sulphates. (Source: IPCC) Large amounts of anthropogenic greenhouse gases have been emitted to the atmosphere since the beginning of the industrial revolution. Since 1750: the carbon dioxide concentration has increased by 31%, methane has increased 151%, nitrous oxide has increased 17% and tropospheric ozone has increased 36%. (Source: IPCC). The majority of the anthropogenic carbon dioxide is produced by the combustion of fossil fuels. Methane is produced by cattle, energy, and rice production in similar amounts, each of which emit about 66% of the amount produced by the major natural source, wetlands[1]. It is thought that the reduction in tropical forested area has also played a role, as old forests store large amounts of carbon.

Carbon Sources and Sinks

Forests which are growing in North America and Russia contribute to absorbing carbon dioxide (they act as CO₂ sinks), and since 1990, the amount of carbon absorbed may be larger than the amount released by deforestation (source???). Conversely, deforestation largely in tropical countries is a source of CO2 to the atmosphere. CO2 releases from deforestation are probably about 1/6 of sources from fossil fuel conbustion (source???).

Not all the CO₂ emitted to the atmosphere accumulates there; half of it is absorbed, presumably by oceans and forests, as a modification to the natural carbon cycle.

Evaluation of the relative importance of various factors

The relative importance of each of the proposed causes varies according to the period of interest: for example, anthropogenic factors are presumed to be negligibly small for climate change before, say, 1750.

Otherwise, their importance can be established through the quantification of the factors involved. Internal factors and the response to external factors can be estimated by the analysis of climate simulations based on the best climate models.

Attribution of climate change

See: Anthropogenic global warming

The most fiercely-contested question in current climate change research is over attribution of climate change to either natural/internal or human factors over the period of the instrumental record - from about 1860, and especially over the last 50 years. In the 1995 SAR the IPCC made the widely quoted statement that "The balance of evidence suggests a discernible human influence on global climate”. The phrase "balance of evidence" was used deliberately to suggest the (English?) common-law standard of proof required in civil as opposed to criminal courts: not as high as "beyond reasonable doubt". In 2001 the TAR upgraded this by saying "There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities" [1].

Over the past 5 decades there has been a warming of approximately 0.3 oC at the Earth's surface (see historical temperature record. This warming might have been caused by internal variability, or by external forcing, or by "greenhouse" gases. Current studies indicate the latter is most likely, on the grounds that

• estimates of internal variability from climate models, and reconstructions of past temperatures, indicate that the warming is unlikely to be entirely natural;
• climate models, forced by changes in greenhouse gases and aerosols, reproduce the observed global changes; those forced by natural factors alone do not;
• "fingerprint" methods indicate that the pattern of change is closer to that expected from GHG forced change than from natural change [1].

However, there is room for disagreement, both about the magnitude of the observed temperature changes and the certainty of the attribution. References to the scientific literature may be found within the IPCC TAR, especially section [2.3] and chapter [12].

General Agreement

There is general agreement among scientists (as revealed by the scientific literature); refs...) that:

• The 100 kyr ice age cycles are controlled by orbital forcing - variations in the seasonal and geographical distribution of insolation; and in the tota insolation.
• The rapid temperature changes seen in ice cores during the last glacial were probably caused by events associated with the Laurentide ice sheet and thus count as "internal variability".
• The Little Ice Age was probably caused by solar variation or volcanic activity.
• The majority of the warming in the last 3-4 decades is anthropogenic.

Areas of disagreement

There are, however, scientists such as Willie Soon and Richard Lindzen who say that there is insufficient proof of this as yet - though they have not written papers showing this.

Six editors of the journal Climate Research resigned in protest after the journal published the article "Proxy Climatic and Environmental Changes of the Past 1,000 Years," by W. Soon and S. Baliunas. Note that this article was not about attribution of climate change.

See:

• global warming, global warming controversy
• Willie Soon, Sallie Baliunas

Global warming episodes in the geological record

Permian-Triassic extinction event

Paleocene-Eocene Thermal Maximum

See also

• Climate model
• global warming