Steven C. Sherwood

Spread in model climate sensitivity traced to atmospheric convective mixing

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.

An adaptability limit to climate change due to heat stress

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.

A Cloud and Precipitation Feature Database from Nine Years of TRMM Observations

An event-based method of analyzing the measurements from multiple satellite sensors is presented by using observations of the Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR), Microwave Imager (TMI), Visible and Infrared Scanner (VIRS), and Lightning Imaging System (LIS). First, the observations from PR, VIRS, TMI, and LIS are temporally and spatially collocated. Then the cloud and precipitation features are defined by grouping contiguous pixels using various criteria, including surface rain, cold infrared, or microwave brightness temperature. The characteristics of measurements from different sensors inside these features are summarized. Then, climatological descriptions of many properties of the identified features are generated. This analysis method condenses the original information of pixellevel measurements into the properties of events, which can greatly increase the efficiency of searching and sorting the observed historical events. Using the TRMM cloud and precipitation feature database, the regional variations of rainfall contribution by features with different size, intensity, and PR reflectivity vertical structure are shown. Above the freezing level, land storms tend to have larger 20-dBZ area and reach higher altitude than is the case for oceanic storms, especially those land storms over central Africa. Horizontal size and the maximum reflectivity of oceanic storms decrease with altitude. For land storms, these intensity measures increase with altitude between 2 km and the freezing level and decrease more slowly with altitude above the freezing level than for ocean storms.

Tropospheric water vapor, convection, and climate

[1] Recent progress is reviewed in the understanding of convective interaction with water vapor and changes associated with water vapor in warmer climates. Progress includes new observing techniques (including isotopic methods) that are helping to illuminate moisture‐convection interaction, better observed humidity trends, new modeling approaches, and clearer expectations as to the hydrological consequences of increased specific humidity in a warmer climate. A theory appears to be in place to predict humidity in the free troposphere if winds are known at large scales, providing a crucial link between small‐scale behavior and large‐scale mass and energy constraints. This, along with observations, supports the anticipated water vapor feedback on climate, though key uncertainties remain connected to atmospheric dynamics and the hydrological consequences of a moister atmosphere. More work is called for to understand how circulations on all scales are governed and what role water vapor plays. Suggestions are given for future research.

On the control of stratospheric humidity

We present a hypothesis on the dehydration and transfer of air from the tropical troposphere into the stratosphere. The hypothesis is based on the existence of a thick “tropopause layer,” in which vertical and horizontal mixing are both significant. Air is rapidly dehydrated upon entering this layer in vigorous convective overshoots, then slowly ascends through the layer before fully entering the stratosphere. Dehydration and genuine entry into the stratosphere are separate processes that happen on much different time scales.

A Drier Future

Global warming is likely to lead to overall drying of land surfaces. Global temperature increases affect the water cycle over land, but the nature of these changes remains difficult to predict. A key conceptual problem is to distinguish between droughts, which are transient regional extreme phenomena typically defined as departures from a local climatological norm that is presumed known, and the normal or background dryness itself. This background dryness depends on precipitation, but also on how fast water would evaporate. As the planet warms, global average rainfall increases, but so does evaporation. What is the likely net impact on average aridity?

A Model for Transport across the Tropical Tropopause

A model of convective and advective transport across the tropical tropopause is described. In this model overshooting convective turrets inject dehydrated tropospheric air into a tropical ‘‘tropopause layer’’ (TTL) bounded approximately by the 50- and 150-hPa surfaces, a layer similar to the ‘‘entrainment zone’’ at the top of the planetary boundary layer. The overshooting process occurs only in limited regions. In the TTL, mixtures of overshooting and ambient air undergo buoyancy-driven settling, then slowly loft through the TTL and eventually enter the main stratosphere throughout the Tropics. It is found that for reasonable parameter settings the combined action of convection, isentropic mixing, and advection by the large-scale circulation in the model can produce realistic water vapor and ozone profiles while balancing the energy budget. Some of the observed peculiarities that can be simulated are (i) the widespread absence of vapor saturation at the tropopause despite tropical mean upward motion, (ii) an ozone minimum below the mean tropopause, and (iii) the typical location of stratiform cloud tops below the mean tropopause. In contrast to inferences from typical ‘‘cold trap’’ models, the relative humidity of air crossing the tropopause is found to be sensitive to ice microphysics.

Convective Precursors and Predictability in the Tropical Western Pacific

Abstract Conditions leading to convective outbreak in the Tropics are investigated by multivariate analysis of sounding and satellite data from the tropical western Pacific area. Circumstances that make the prediction problem difficult are discussed and addressed by applying linear “error-in-variables” and nonlinear statistical simulation techniques to a large dataset. Low- to midtropospheric moisture is identified as the dominant factor regulating convective outbreak in this region. Based on the results it is argued that such moisture is particularly important in regulating spontaneous convective outbreak, but instability and near-surface wind speed probably play some role in allowing previous shallow or midtopped cumulus activity to deepen. Mesoscale-mean convective available potential energy sufficient for convection is found to exist almost 90% of the time. Quantitative estimates of noise in the data are obtained and accounted for in reaching these conclusions. The results imply that large-scale mean ...

Recent Northern Hemisphere tropical expansion primarily driven by black carbon and tropospheric ozone.

Observational analyses have shown the width of the tropical belt increasing in recent decades as the world has warmed. This expansion is important because it is associated with shifts in large-scale atmospheric circulation and major climate zones. Although recent studies have attributed tropical expansion in the Southern Hemisphere to ozone depletion, the drivers of Northern Hemisphere expansion are not well known and the expansion has not so far been reproduced by climate models. Here we use a climate model with detailed aerosol physics to show that increases in heterogeneous warming agents—including black carbon aerosols and tropospheric ozone—are noticeably better than greenhouse gases at driving expansion, and can account for the observed summertime maximum in tropical expansion. Mechanistically, atmospheric heating from black carbon and tropospheric ozone has occurred at the mid-latitudes, generating a poleward shift of the tropospheric jet, thereby relocating the main division between tropical and temperate air masses. Although we still underestimate tropical expansion, the true aerosol forcing is poorly known and could also be underestimated. Thus, although the insensitivity of models needs further investigation, black carbon and tropospheric ozone, both of which are strongly influenced by human activities, are the most likely causes of observed Northern Hemisphere tropical expansion.

Climate Effects of Aerosol-Cloud Interactions

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 Earths 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.