Climate Change 2001: Synthesis Report

Other reports in this collection

Question 4

What is known about the influence of the increasing atmospheric concentrations of greenhouse gases and aerosols, and the projected human-induced change in climate regionally and globally on:

  1. The frequency and magnitude of climate fluctuations, including daily, seasonal, inter-annual, and decadal variability, such as the El Niño Southern Oscillation cycles and others?
  2. The duration, location, frequency, and intensity of extreme events such as heat waves, droughts, floods, heavy precipitation, avalanches, storms, tornadoes, and tropical cyclones?
  3. The risk of abrupt/non-linear changes in, among others, the sources and sinks of greenhouse gases, ocean circulation, and the extent of polar ice and permafrost? If so, can the risk be quantified?
  4. The risk of abrupt or non-linear changes in ecological systems?

An increase in climate variability and some extreme events is projected.


Models project that increasing atmospheric concentrations of greenhouse gases will result in changes in daily, seasonal, inter-annual, and decadal variability. There is projected to be a decrease in diurnal temperature range in many areas, decrease of daily variability of surface air temperature in winter, and increased daily variability in summer in the Northern Hemisphere land areas. Many models project more El Niño-like mean conditions in the tropical Pacific. There is no clear agreement concerning changes in frequency or structure of naturally occurring atmosphere-ocean circulation patterns such as that of the North Atlantic Oscillation (NAO)

Models project that increasing atmospheric concentrations of greenhouse gases result in changes in frequency, intensity, and duration of extreme events, such as more hot days, heat waves, heavy precipitation events, and fewer cold days. Many of these projected changes would lead to increased risks of floods and droughts in many regions, and predominantly adverse impacts on ecological systems, socio-economic sectors, and human health (see Table SPM-2 for details). High resolution modeling studies suggest that peak wind and precipitation intensity of tropical cyclones are likely to increase over some areas. There is insufficient information on how very small-scale extreme weather phenomena (e.g., thunderstorms, tornadoes, hail, hailstorms, and lightning) may change. Q4.2-7

Greenhouse gas forcing in the 21st century could set in motion large-scale, high-impact, non-linear, and potentially abrupt changes in physical and biological systems over the coming decades to millennia, with a wide range of associated likelihoods.


Some of the projected abrupt/non-linear changes in physical systems and in the natural sources and sinks of greenhouse gases could be irreversible, but there is an incomplete understanding of some of the underlying processes. The likelihood of the projected changes is expected to increase with the rate, magnitude, and duration of climate change.

Examples of these types of changes include:

  • Large climate-induced changes in soils and vegetation may be possible and could induce further climate change through increased emissions of greenhouse gases from plants and soil, and changes in surface properties (e.g., albedo).
  • Most models project a weakening of the thermohaline circulation of the oceans resulting in a reduction of heat transport into high latitudes of Europe, but none show an abrupt shutdown by the end of the 21st century. However, beyond the year 2100, some models suggest that the thermohaline circulation could completely, and possibly irreversibly, shut down in either hemisphere if the change in radiative forcing is large enough and applied long enough.
  • The Antarctic ice sheet is likely to increase in mass during the 21st century, but after sustained warming the ice sheet could lose significant mass and contribute several meters to the projected sea-level rise over the next 1,000 years.
  • In contrast to the Antarctic ice sheet, the Greenland ice sheet is likely to lose mass during the 21st century and contribute a few cm to sea-level rise. Ice sheets will continue to react to climate warming and contribute to sea-level rise for thousands of years after climate has been stabilized. Climate models indicate that the local warming over Greenland is likely to be one to three times the global average. Ice sheet models project that a local warming of larger than 3°C, if sustained for millennia, would lead to virtually a complete melting of the Greenland ice sheet with a resulting sea-level rise of about 7 m. A local warming of 5.5°C, if sustained for 1,000 years, would likely result in a contribution from Greenland of about 3 m to sea-level rise.
  • Continued warming would increase melting of permafrost in polar, sub-polar, and mountain regions and would make much of this terrain vulnerable to subsidence and landslides which affect infrastructure, water courses, and wetland ecosystems.
Table SPM-2: Examples of climate variability and extreme climate events and examples of their impacts (WGII TAR Table SPM-1).
Projected Changes during the 21st Century in Extreme Climate Phenomena and their Likelihood Representative Examples of Projected Impactsa
(all high confidence of occurrence in some areas)
Higher maximum temperatures, more hot days and heat wavesb over nearly all land areas (very likely) Increased incidence of death and serious illness in older age groups and urban poor.
Increased heat stress in livestock and wildlife.
Shift in tourist destinations.
Increased risk of damage to a number of crops.
Increased electric cooling demand and reduced energy supply reliability.
Higher (increasing) minimum temperatures, fewer cold days, frost days and cold wavesb over nearly all land areas (very likely) Decreased cold-related human morbidity and mortality.
Decreased risk of damage to a number of crops, and increased risk to others.
Extended range and activity of some pest and disease vectors.
Reduced heating energy demand.
More intense precipitation events (very likely, over many areas) Increased flood, landslide, avalanche, and mudslide damage.
Increased soil erosion.
Increased flood runoff could increase recharge of some floodplain aquifers.
Increased pressure on government and private flood insurance systems and disaster relief.
Increased summer drying over most midlatitude continental interiors and associated risk of drought (likely) Decreased crop yields.
Increased damage to building foundations caused by ground shrinkage.
Decreased water resource quantity and quality.
Increased risk of forest fire.
Increase in tropical cyclone peak wind intensities, mean and peak precipitation intensities (likely, over some areas)c Increased risks to human life, risk of infectious disease epidemics and many other risks.
Increased coastal erosion and damage to coastal buildings and infrastructure.
Increased damage to coastal ecosystems such as coral reefs and mangroves.
Intensified droughts and floods associated with El Niño events in many different regions (likely) (see also under droughts and intense precipitation events) Decreased agricultural and rangeland productivity in drought- and flood-prone regions.
Decreased hydro-power potential in drought-prone regions.
Increased Asian summer monsoon precipitation variability (likely) Increase in flood and drought magnitude and damages in temperate and tropical Asia.
Increased intensity of mid-latitude storms (little agreement between current models)b Increased risks to human life and health.
Increased property and infrastructure losses.
Increased damage to coastal ecosystems.
a. These impacts can be lessened by appropriate response measures.
b. Information from WGI TAR Technical Summary (Section F.5).
c. Changes in regional distribution of tropical cyclones are possible but have not been established.

Changes in climate could increase the risk of abrupt and non-linear changes in many ecosystems, which would affect their function, biodiversity, and productivity. The greater the magnitude and rate of the change, the greater the risk of adverse impacts. For example:

  • Changes in disturbance regimes and shifts in the location of suitable climatically defined habitats may lead to abrupt breakdown of terrestrial and marine ecosystems with significant changes in composition and function and increased risk of extinctions.
  • Sustained increases in water temperatures of as little as 1°C, alone or in combination with any of several stresses (e.g., excessive pollution and siltation), can lead to corals ejecting their algae (coral bleaching) and the eventual death of some corals.
  • Temperature increase beyond a threshold, which varies by crop and variety, can affect key development stages of some crops (e.g., spikelet sterility in rice, loss of pollen viability in maize, tubers development in potatoes) and thus the crop yields. Yield losses in these crops can be severe if temperatures exceed critical limits for even short periods.

Other reports in this collection

IPCC Homepage