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)

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.

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.