Recent heat extremes, wildfires, and droughts have afflicted multiple regions including
parts of South America, the Mediterranean, and Middle East as well as the southwestern
United States. These events underscore the vulnerability of societies to ongoing climate
change as well as the precarious dependence of societies and ecosystems upon fresh
water, the terrestrial lifeblood. A warming climate is projected to generate more
intense wet as well as dry periods, and this is based on multiple lines of evidence
(1
–3) including computer simulations built with the most comprehensive representation
of physics applied to the land, air, and sea (4). Long-term continental drying has
previously been identified in terms of increasing aridity, reducing soil moisture,
and also a decline in atmospheric relative humidity, a measure of how close to saturation
the air is with gaseous water vapor (1). Greater warming over land than ocean is an
expected consequence of global warming and implicated in continental relative humidity
decline since the oceans are unable to supply enough moisture to maintain the levels
of saturation over the hotter land (5). Building on previous findings of an overall
underestimate in continental drying by climate models (6, 7) a new study in PNAS (8)
exploits the most up-to-date observations and simulations to expound this discrepancy
between models and observations: they identify a decline in water vapor over many
arid and semi-arid continental regions that is contrary to expectations. This has
implications for these already vulnerable regions which could experience an even drier
future than predicted in response to global warming.
Basic physics dictates that the atmosphere’s thirst for water grows with warming,
driving an increase in the gaseous form of water in the atmosphere. Through its ability
as a potent greenhouse gas to absorb outgoing infrared radiation, as well as incoming
sunlight, the increases in water vapor add to the temperature rise through a reenforcing
feedback loop. A warmer, thirstier atmosphere can also more effectively sap moisture
from the land or ocean in one region and transport this extra water into storm systems
elsewhere, leading to intensification of heavy rainfall events as well as promoting
more rapid onset of dry spells, with implications for worsening flooding as well as
drought (1). It is therefore of utmost importance to establish fidelity in the processes
determining water vapor changes in a warming climate. Although there are multiple
lines of evidence to support the global increase in atmospheric moisture as the planet
heats up (9, 10), the new research (8) finds compelling evidence of a deficiency in
how climate models represent low-level moisture changes above many arid and partially
arid regions across the globe.
Assessing simulated changes in surface water vapor requires scrutiny of a comprehensive
set of observations from ground-based meteorological sites. Ensuring the fidelity
of long-term surface humidity records is a particular challenge, requiring rigorous
account for biases due, for example, to daytime solar heating of poorly ventilated
instruments, as well as changes in observing practice and reporting (11). Presenting
a convincing case that the measurements are robust, Simpson et al. (8) combine these
records with a state-of-the-art global reanalysis (12) which uses multiple data sources
to ensure that a complex physics-based global simulation remains as close as possible
to reality.
As an illustrative case study, the southwestern United States is highlighted as a
key region exhibiting drying trends that is well endowed with reliable observations.
The finding that water vapor amount has been declining rather than increasing here
since the 1980s applies more broadly to drier climates globally (e.g., northern Argentina,
southern Africa, parts of East Africa, western Mediterranean, central Asia, and eastern
Australia) as well as to dry seasons in less arid climates. In contrast, climate models
generally simulate increases in water vapor (Fig. 1) and even the most desiccating
scenarios from the range of simulations considered fall short of the observed decline
in atmospheric moisture over arid and semi-arid regions (8).
Fig. 1.
Changes in 2 m (near surface) specific humidity (q) and column integrated water vapor
(CWV) from the 1988 to 2003 period to the 2004 to 2019 period from the ECWMF fifth
generation reanalysis (ERA5; 12) (A and B) and two simulations (C–F) from the sixth
phase of the Coupled Model Intercomparison Project (CMIP6; 4) combining historical
and ssp2-4.5 (an intermediate greenhouse gas emissions scenario) experiments. The
CanESM5 simulations produce a larger increase in moisture than the CNRM-CM6-1 simulation
but neither are able to capture the magnitude or extent of decline in surface specific
humidity (A) over arid and semi-arid regions such as the southwestern United States,
central and southern South America, southern Africa, and Australia.
One suggested cause for the discrepancy between observed and simulated climate change
is that the magnitude and pattern of global warming has diverged from the free-running
simulations that generate their own climate variability through ocean and atmospheric
fluctuations (13). Indeed, when the observed ocean warming pattern is fed into the
model experiments, the discrepancy in continental drying diminishes, yet most simulations
remain unable to capture the magnitude of drying, pointing to a more fundamental deficiency
in model physics.
The drying signal is most acute at the lowest altitudes above the land surface (8,
9); this may provide a clue as to the causes of differences compared to model simulations
of the historical period. Changing wind patterns, due to natural variations, or driven
by longer-term climate change, can contribute to drying in some regions. However,
accounting for possible changes in moisture-laden winds by selecting locations where
precipitation has remained largely unchanged, Simpson et al. find over half of these
dry areas exhibit reduced atmospheric moisture, in contrast to only about 10% or less
of comparable regions in the simulations. This controlling for rainfall changes may
suggest a more prominent role for evaporation, or the recycling of rainfall supplied
by moisture transported by the winds (14, 15). Soil moisture has decreased in many
regions over recent decades, particularly in the southern hemisphere (16), and these
emerging trends in terrestrial water storage can be linked to multiple drivers including
natural variability as well as human-caused climate change or direct water use (17,
18). However, more numerous dry days combined with an increased evaporative demand
are likely to contribute to the observed subtropical drying through interaction between
the land surface and the atmosphere (19).
Although the cause of the discrepancy in atmospheric humidity trends is unclear, Simpson
et al. offer several plausible explanations: the ground may not have dried out as
much as the real world; models may be leaking unrealistic amounts of moisture into
the air; the partitioning of rainfall into runoff, soil moisture, or recycled back
into the air through evaporation may be incorrect; or plants could be withholding
their water as a result of their stomatal responses to rising levels of carbon dioxide
(20), to an even greater extent than the models predict. Since the simulations are
able to resolve the large-scale wind patterns and basic thermodynamic processes determining
moisture supply, it seems more likely that processes operating at the land–atmosphere
interface are involved in explaining the difference to the observed humidity changes.
In particular, the fine detail characteristics of rainfall intermittency in space
and time are not explicitly represented by the models and how precipitation is partitioned
between runoff, infiltration, evaporation from the ground and transpiration from plants
may be unrealistically simulated over drier climates. Land surface feedbacks involving
soil moisture and vegetation are known to amplify continental drying (21) but multiple,
interconnected processes need to be realistically modeled to obtain reliable simulations.
The implications of enhanced continental drying are substantial. Even drier arid zones
will put further pressure on water resources and intensify extreme heat and wildfires.
Beyond the subtropics, drier dry seasons in the tropics and mid-latitudes may also
lead to damaging extreme events, as experienced across the globe in the early 2020s.
A larger than simulated recent near-surface drying over arid regions may also imply
a stronger than projected future continental warming (18). An even thirstier atmosphere
can also lead to more rapid onset of drought that is already projected to intensify
(22) while the greater swings between wet and dry (1
–3) present a challenge for water resource planning which remains a neglected consideration
in both mitigation and adaptation options (23). It is therefore crucial to fully understand
why climate model simulations appear to underestimate drying over arid and partially
arid regions. Although scrutiny of the observational records remains necessary to
further confirm and characterize the continental drying, understanding differences
with climate model simulations will require detailed and high-resolution regional
modeling experiments to identify and correct deficient processes relating to rainfall
characteristics and how this water is apportioned between the ground, vegetation,
and the air. Ultimately, a potentially drier-than-anticipated future over arid regions
will also require more extensive adaptation plans.
A new study by Simpson et al. exploits the most up to date observations and simulations
to elucidate a discrepancy between models and observations: they identify a decline
in water vapor over many arid and semi-arid continental regions that is contrary to
expectations.