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ACLH Key Findings
ABRUPT CHANGES IN THE EARTH’S CLIMATE SYSTEM
ABRUPT CHANGE IN LAND HYDROLOGY
- Protracted droughts, and their impacts on agricultural production and water supplies, are among the greatest natural hazards facing the United States and the globe today and in the foreseeable future.
- Floods predominantly reflect both antecedent conditions and meteorological events and are often more localized relative to drought in both time and space. On subcontinental-to-continental scales, droughts occur more frequently than floods and can persist for decades and even centuries.
- On interannual to decadal time scales, droughts can develop faster than the time scale needed for human societies to adapt to the change. Thus, a severe drought lasting several years can be regarded as an abrupt change, although it may not reflect a permanent change of state of the climate system.
- Droughts and episodes of regional-scale flooding can both be linked to the large-scale atmospheric circulation patterns over North America, and often occur simultaneously in different parts of the country, compounding their impact on human activities.
- Empirical studies and climate model experiments conclusively show that droughts over North America have been significantly influenced by the state of tropical sea-surface temperatures (SSTs). Of particular relevance to North America, cool La Niña-like SSTs in the eastern equatorial Pacific frequently cause development of droughts over the Southwestern United States and Northern Mexico. Warm subtropical North Atlantic SSTs play a secondary role in forcing drought in southwestern North America.
- Historic droughts over North America have been severe, the "Dust Bowl” drought of the 1930s being the canonical example, but those droughts were not nearly as prolonged as a series of "megadroughts” reconstructed from tree rings since Medieval times (ca. 1,000 years ago) up to about A.D. 1600. Modeling experiments indicate that these megadroughts were likely partly forced by cool SSTs in the eastern equatorial Pacific as well. However, their exceptional duration has not been adequately explained nor has any involvement in forcing from SST changes in other oceans.
- These megadroughts are significant because they occurred in a climate system that was not being perturbed in a major way by human activity (i.e., the ongoing anthropogenic changes in greenhouse gas concentrations, atmospheric dust loadings, and land-cover changes).
- Even larger and more persistent changes in hydroclimatic variability worldwide are indicated throughout the Holocene (the past 11,500 years) by a diverse set of paleoclimatic indicators including some with annual-to-decadal resolution (e.g., speleothems, varved-lake records, high-resolution lake-sediment records). The global-scale controls associated with those changes were quite different from those of the past millennium and today, but they show the additional range of natural variability and abrupt hydroclimatic change that can be expressed by the climate system, including widespread and protracted (multi-century) droughts.
- There is no clear evidence to date of human-induced global climate change on North American precipitation amounts. However, since the IPCC AR4 report, further analysis of climate model scenarios of future hydroclimatic change over North America and the global subtropics indicates that subtropical aridity is likely to intensify and persist due to future greenhouse warming. This projected drying extends poleward into the United States Southwest, potentially increasing the likelihood of severe and persistent drought there in the future. If the model results are correct, then this drying may have already begun, but currently cannot be definitively identified amidst the considerable natural variability of hydroclimate in Southwestern North America.
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