ABRUPT CHANGES IN THE EARTH’S CLIMATE SYSTEM
ABRUPT CHANGE IN LAND HYDROLOGY
Key Findings
- 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.
