Despite the uncertainty in future climate-change impacts, it is often assumed that humans would be able to adapt to any possible warming. Here we argue that heat stress imposes a robust upper limit ...to such adaptation. Peak heat stress, quantified by the wet-bulb temperature TW, is surprisingly similar across diverse climates today. TW never exceeds 31 °C. Any exceedence of 35 °C for extended periods should induce hyperthermia in humans and other mammals, as dissipation of metabolic heat becomes impossible. While this never happens now, it would begin to occur with global-mean warming of about 7 °C, calling the habitability of some regions into question. With 11-12 °C warming, such regions would spread to encompass the majority of the human population as currently distributed. Eventual warmings of 12 °C are possible from fossil fuel burning. One implication is that recent estimates of the costs of unmitigated climate change are too low unless the range of possible warming can somehow be narrowed. Heat stress also may help explain trends in the mammalian fossil record.
Equilibrium climate sensitivity refers to the ultimate change in global mean temperature in response to a change in external forcing. Despite decades of research attempting to narrow uncertainties, ...equilibrium climate sensitivity estimates from climate models still span roughly 1.5 to 5 degrees Celsius for a doubling of atmospheric carbon dioxide concentration, precluding accurate projections of future climate. The spread arises largely from differences in the feedback from low clouds, for reasons not yet understood. Here we show that differences in the simulated strength of convective mixing between the lower and middle tropical troposphere explain about half of the variance in climate sensitivity estimated by 43 climate models. The apparent mechanism is that such mixing dehydrates the low-cloud layer at a rate that increases as the climate warms, and this rate of increase depends on the initial mixing strength, linking the mixing to cloud feedback. The mixing inferred from observations appears to be sufficiently strong to imply a climate sensitivity of more than 3 degrees for a doubling of carbon dioxide. This is significantly higher than the currently accepted lower bound of 1.5 degrees, thereby constraining model projections towards relatively severe future warming.
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Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
Heuristic models and observational analyses of atmospheric convection often assume that convective activity, for example, rain rate, approaches some given value for any given large-scale ...(“macrostate”) environmental conditions, such as static stability and humidity. We present novel convection-resolving simulations in which the convective activity evolves in a fixed equilibrium mean state (“macrostate”). In this case, convective activity is unstable, diverging quasi exponentially away from equilibrium either to extreme or zero rain rate. Thus, almost any rain rate can coexist with an equilibrium profile: the model rain rate also depends on convective history. We then present a two-variable, predator–prey model motivated by this behavior, wherein small-scale (“microstate”) variability is produced by but also promotes convective precipitation, while macrostate properties such as CAPE promote but are consumed by convective precipitation. In this model, convection is influenced as much by its own history (via persistent microstate variability) as by its current environment. This model reproduces the simulated instability found above and could account for several lag relationships in simulated and observed convection, including its afternoon maximum over land and the well-known “quasi-equilibrium” balance at synoptic time scales between the forcing and response of key variables. These results point to a strong role for convective memory and suggest that basic strategies for observing, modeling, and parameterizing convective processes should pay closer attention to persistent variability on scales smaller than that of the grid box.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
4.
Climate Effects of Aerosol-Cloud Interactions Rosenfeld, Daniel; Sherwood, Steven; Wood, Robert ...
Science (American Association for the Advancement of Science),
01/2014, Letnik:
343, Številka:
6169
Journal Article
Recenzirano
Advances in satellite observations and model development are needed to disentangle the complex interactions of aerosols and clouds and their effects on climate.
Aerosols counteract part of the ...warming effects of greenhouse gases, mostly by increasing the amount of sunlight reflected back to space. However, the ways in which aerosols affect climate through their interaction with clouds are complex and incompletely captured by climate models. As a result, the radiative forcing (that is, the perturbation to Earth's energy budget) caused by human activities is highly uncertain, making it difficult to predict the extent of global warming (
1
,
2
). Recent advances have led to a more detailed understanding of aerosol-cloud interactions and their effects on climate, but further progress is hampered by limited observational capabilities and coarse-resolution climate models.
How Important Is Humidity in Heat Stress? Sherwood, Steven C.
Journal of geophysical research. Atmospheres,
16 November 2018, Letnik:
123, Številka:
21
Journal Article
Recenzirano
Odprti dostop
Recent studies highlight the importance of humidity in accounting for extreme values of wet‐bulb temperature, a proposed indicator of heat stress. But this result may be sensitive to the measure of ...heat stress used—an ongoing source of confusion.
Plain Language Summary
There is confusion about how to measure heat stress, in particular between the wet‐bulb globe temperature often used to screen for dangerous heat at sporting events and in workplaces, the wet‐bulb temperature beloved of weather geeks, and apparent temperature quoted to the public by weather services. Here these distinctions are demystified. The distinction between these quantities matters a lot when considering how important humidity is to heat stress.
Key Points
It is important to distinguish similar‐sounding variables for characterizing heat
Wet‐bulb temperature is very humidity sensitive, WBGT less so, and apparent temperature the least
WBGT is a more appropriate measure of present‐day heat stress, but wet bulb is relevant to tolerance of hotter climates
Abstract
The horizontal temperature gradients in the tropical free troposphere are generally assumed to be weak. We show with ERA5 data that substantial zonal virtual temperature (
T
υ
) gradients ...persist climatologically in the tropical free troposphere and investigate their causes. The gradients change seasonally:
T
υ
at 500 hPa over the equatorial western Pacific Ocean (EWP) is usually much warmer (up to 3 K) than that over the equatorial central Pacific Ocean (ECP) during December–February (DJF), while the temperature differences between EWP and ECP are much smaller during June–August (JJA). During DJF,
T
υ
gradients over the Pacific prevail throughout the entire free troposphere, especially in the upper troposphere near 300 hPa. We find that the associated hydrostatic pressure gradients are mainly balanced by the nonlinear terms in the momentum equation, in particular via zonal wind advection. Strong zonal winds occur near the equator in boreal winter, transporting zonal momentum so as to balance the pressure gradient force. The zonal winds are due to large-scale equatorial waves, excited by a heating pattern that is relatively symmetric about the equator. In boreal summer, the large-scale equatorial waves are less active in the Pacific region due to a more asymmetric temperature pattern, so the zonal momentum advection and
T
υ
gradients are both much weaker. The results point to an important role of the nonlinear terms in the tropical balanced dynamics, stressing the need for an improved theoretical understanding and modeling framework of the tropical atmosphere that includes these nonlinear terms, or their net effect.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
8.
Adapting to the challenges of warming Sherwood, Steven C
Science (American Association for the Advancement of Science),
2020-Nov-13, 2020-11-13, 20201113, Letnik:
370, Številka:
6518
Journal Article
Recenzirano
The impact of regional heat events is becoming more important to quantify
Heat extremes on Earth have reached a disturbing new level in recent years. The July 2020 temperatures soared across Siberia ...and reached a record-breaking 38°C inside the Arctic Circle, continuing a line of record heat events globally. “Event attribution” calculations, which are an endeavor to apportion blame for extreme events through quantitative modeling, suggest that some events would have been nearly impossible without human-induced global warming. This includes the recent Siberian summer and the 2018 heat wave in Japan, which killed more than a thousand people (
1
,
2
). Rising heat is creating new challenges for humanity that will require new adaptation and protection measures. Smart implementation requires careful calculation of how further global temperature rises will translate into short-term regional heat events and how these will translate into impacts on human health and activities, food supply, infrastructure, and ecosystems.
Abstract
Although the steady, entraining, updraft plume is widely taken as the foundational concept of cumulus convection, past studies show that convection is typically dominated by thermals that ...are transient, more isotropic in shape, and possess interior vortical circulations. Here, several thousand such thermals are tracked in cloud-resolving simulations of transient growing convective events. Most tracked thermals are small (with radius R < 300 m), ascend at moderate rates (~ 2–4 m s−1), maintain an approximately constant size as they rise, and have brief (4–5 min) lifetimes, although a few are much larger, faster, and/or longer lived. They show slight vertical elongation, but few, if any, would be described as plumes. As convection deepens, thermals originate higher up, are larger, and rise faster, although radius and ascent rate are only weakly correlated among individual thermals. The main force opposing buoyancy is a nonhydrostatic pressure drag, not mixing of momentum. This drag can be expressed in terms of a drag coefficient cd that decreases as convection intensifies: deep convective thermals are less damped, with cd ~ 0.2, while shallow convective thermals are more damped, with cd ~ 0.6. The expected dependence of cd based on theoretical form and wave drag coefficients for a solid sphere is inconsistent with these results, since it predicts the opposite dependence on the Froude number. Thus, a theory for drag on cumulus thermals is not straightforward. Overall, it is argued that thermals are a more realistic prototype for atmospheric deep convection than plumes, at least for the less organized convection types simulated here.
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Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
Mixing is one of the most important processes associated with atmospheric moist convection. It determines the two-way interaction between clouds and their environment, thus having a direct ...impact on the time evolution of convection. The fractional entrainment rate ε—the main parameter related to mixing—is often parameterized in global circulation models as a function of updraft properties, and at the same time has a strong influence on how convection evolves. Within the framework of cumulus thermal vortices in large-eddy simulations of convection, here we first investigate the validity of some of the most common parameterizations of ε, and then investigate how relevant ε is for the fate of these thermals. We find that 1/R, where R is a measure of the thermal’s radius, best parameterizes ε, but it explains only about 20% of the total variance. On the other hand, we find that both ε and favorable initial conditions—including high initial saturated fraction of the thermals—are key factors that affect the thermals’ ascent rate, mean buoyancy, and distance traveled. The lifetimes of thermals, however, seem not to be affected significantly by either ε or initial conditions, which supports the view of cumulus convection as a succession of many short-lived thermals. Finally, our results suggest that for the majority of in-cloud cumulus thermals the important role of environmental moisture in the deepening of convection results mainly from providing the initial moisture for the short-lived thermals as they initiate at different altitudes above cloud base, rather than favoring their buoyancy as they rise through it.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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