Changes have also occurred in other important aspects of climate.

• It is very likely⁷ that precipitation has increased by 0.5 to 1% per decade in the 20th century over most mid- and high latitudes of the Northern Hemisphere continents, and it is likely⁷ that rainfall has increased by 0.2 to 0.3% per decade over the tropical (10°N to 10°S) land areas. Increases in the tropics are not evident over the past few decades. It is also likely⁷ that rainfall has decreased over much of the Northern Hemisphere sub-tropical (10°N to 30°N) land areas during the 20th century by about 0.3% per decade. In contrast to the Northern Hemisphere, no comparable systematic changes have been detected in broad latitudinal averages over the Southern Hemisphere. There are insufficient data to establish trends in precipitation over the oceans.
• In the mid- and high latitudes of the Northern Hemisphere over the latter half of the 20th century, it is likely⁷ that there has been a 2 to 4% increase in the frequency of heavy precipitation events. Increases in heavy precipitation events can arise from a number of causes, e.g., changes in atmospheric moisture, thunderstorm activity and large-scale storm activity.
• It is likely⁷ that there has been a 2% increase in cloud cover over mid- to high latitude land areas during the 20th century. In most areas the trends relate well to the observed decrease in daily temperature range.
• Since 1950 it is very likely⁷ that there has been a reduction in the frequency of extreme low temperatures, with a smaller increase in the frequency of extreme high temperatures.
  
• Warm episodes of the El Niño-Southern Oscillation (ENSO)

  Figure 2: Long records of past changes in atmospheric composition provide the context for the influence of anthropogenic emissions. (a) shows changes in the atmospheric concentrations of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) over the past 1000 years. The ice core and firn data for several sites in Antarctica and Greenland (shown by different symbols) are supplemented with the data from direct atmospheric samples over the past few decades (shown by the line for CO2 and incorporated in the curve representing the global average of CH4). The estimated positive radiative forcing of the climate system from these gases is indicated on the right-hand scale. Since these gases have atmospheric lifetimes of a decade or more, they are well mixed, and their concentrations reflect emissions from sources throughout the globe. All three records show effects of the large and increasing growth in anthropogenic emissions during the Industrial Era. (b) illustrates the influence of industrial emissions on atmospheric sulphate concentrations, which produce negative radiative forcing. Shown is the time history of the concentrations of sulphate, not in the atmosphere but in ice cores in Greenland (shown by lines; from which the episodic effects of volcanic eruptions have been removed). Such data indicate the local deposition of sulphate aerosols at the site, reflecting sulphur dioxide (SO2) emissions at mid-latitudes in the Northern Hemisphere. This record, albeit more regional than that of the globally-mixed greenhouse gases, demonstrates the large growth in anthropogenic SO2 emissions during the Industrial Era. The pluses denote the relevant regional estimated SO2 emissions (right-hand scale). [Based upon (a) Chapter 3, Figure 3.2b (CO2); Chapter 4, Figure 4.1a and b ( CH4) and Chapter 4, Figure 4.2 (N2O) and (b) Chapter 5, Figure 5.4a]

  phenomenon (which consistently affects regional variations of precipitation and temperature over much of the tropics, sub-tropics and some mid-latitude areas) have been more frequent, persistent and intense since the mid-1970s, compared with the previous 100 years.
• Over the 20th century (1900 to 1995), there were relatively small increases in global land areas experiencing severe drought or severe wetness. In many regions, these changes are dominated by inter-decadal and multi-decadal climate variability, such as the shift in ENSO towards more warm events.
• In some regions, such as parts of Asia and Africa, the frequency and intensity of droughts have been observed to increase in recent decades.
  

Some important aspects of climate appear not to have changed.

• A few areas of the globe have not warmed in recent decades, mainly over some parts of the Southern Hemisphere oceans and parts of Antarctica.
• No significant trends of Antarctic sea-ice extent are apparent since 1978, the period of reliable satellite measurements.
• Changes globally in tropical and extra-tropical storm intensity and frequency are dominated by inter-decadal to multi-decadal variations, with no significant trends evident over the 20th century. Conflicting analyses make it difficult to draw definitive conclusions about changes in storm activity, especially in the extra-tropics.
• No systematic changes in the frequency of tornadoes, thunder days, or hail events are evident in the limited areas analysed.
  

Emissions of greenhouse gases and aerosols due to human activities continue to alter the atmosphere in ways that are expected to affect the climate.

Changes in climate occur as a result of both internal variability within the climate system and external factors (both natural and anthropogenic). The influence of external factors on climate can be broadly compared using the concept of radiative forcing⁸. A positive radiative forcing, such as that produced by increasing concentrations of greenhouse gases, tends to warm the surface. A negative radiative forcing, which can arise from an increase in some types of aerosols (microscopic airborne particles) tends to cool the surface. Natural factors, such as changes in solar output or explosive volcanic activity, can also cause radiative forcing. Characterisation of these climate forcing agents and their changes over time (see Figure 2) is required to understand past climate changes in the context of natural variations and to project what climate changes could lie ahead. Figure 3 shows current estimates of the radiative forcing due to increased concentrations of atmospheric constituents and other mechanisms.

Concentrations of atmospheric greenhouse gases and their radiative forcing have continued to increase as a result of human activities.

• The atmospheric concentration of carbon dioxide (CO₂) has increased by 31% since 1750. The present CO₂ concentration has not been exceeded during the past 420,000 years and likely⁷ not during the past 20 million years. The current rate of increase is unprecedented during at least the past 20,000 years.
• About three-quarters of the anthropogenic emissions of CO₂ to the atmosphere during the past 20 years is due to fossil fuel burning. The rest is predominantly due to land-use change, especially deforestation.
• Currently the ocean and the land together are taking up about half of the anthropogenic CO₂ emissions. On land, the uptake of anthropogenic CO₂ very likely⁷ exceeded the release of CO₂ by deforestation during the 1990s.
• The rate of increase of atmospheric CO₂ concentration has been about 1.5 ppm⁹ (0.4%) per year over the past two decades. During the 1990s the year to year increase varied from 0.9 ppm (0.2%) to 2.8 ppm (0.8%). A large part of this variability is due to the effect of climate variability (e.g., El Niño events) on CO₂ uptake and release by land and oceans.
• The atmospheric concentration of methane (CH₄) has increased by 1060 ppb⁹ (151%) since 1750 and continues to increase. The present CH₄ concentration has not been exceeded during the past 420,000 years. The annual growth in CH₄ concentration slowed and became more variable in the 1990s, compared with the 1980s. Slightly more than half of current CH₄ emissions are anthropogenic (e.g., use of fossil fuels, cattle, rice agriculture and landfills). In addition, carbon monoxide (CO) emissions have recently been identified as a cause of increasing CH₄ concentration.
• The atmospheric concentration of nitrous oxide (N₂O) has increased by 46 ppb (17%) since 1750 and continues to increase. The present N₂O concentration has not been exceeded during at least the past thousand years. About a third of current N₂O emissions are anthropogenic (e.g., agricultural soils, cattle feed lots and chemical industry).
• Since 1995, the atmospheric concentrations of many of those halocarbon gases that are both ozone-depleting and greenhouse gases (e.g., CFCl₃ and CF₂Cl₂), are either increasing more slowly or decreasing, both in response to reduced emissions under the regulations of the Montreal Protocol and its Amendments. Their substitute compounds (e.g., CHF₂Cl and CF₃CH₂F) and some other synthetic compounds (e.g., perfluorocarbons (PFCs) and sulphur hexafluoride (SF₆)) are also greenhouse gases, and their concentrations are currently increasing.
• The radiative forcing due to increases of the well-mixed greenhouse gases from 1750 to 2000 is estimated to be 2.43 Wm^-2: 1.46 Wm^-2 from CO₂; 0.48 Wm^-2 from CH₄; 0.34 Wm^-2 from the halocarbons; and 0.15 Wm^-2 from N₂O. (See Figure 3, where the uncertainties are also illustrated.)
• The observed depletion of the stratospheric ozone (O₃) layer from 1979 to 2000 is estimated to have caused a negative radiative forcing (–0.15 Wm^-2). Assuming full compliance with current halocarbon regulations, the positive forcing of the halocarbons will be reduced as will the magnitude of the negative forcing from stratospheric ozone depletion as the ozone layer recovers over the 21st century.
• The total amount of O₃ in the troposphere is estimated to have increased by 36% since 1750, due primarily to anthropogenic emissions of several O₃-forming gases. This corresponds to a positive radiative forcing of 0.35 Wm^-2. O₃ forcing varies considerably by region and responds much more quickly to changes in emissions than the long-lived greenhouse gases, such as CO₂.
  

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