Rapid adjustments are responses to forcing agents that cause a perturbation to the top of atmosphere energy budget but are uncoupled to changes in surface warming. Different mechanisms are ...responsible for these adjustments for a variety of climate drivers. These remain to be quantified in detail. It is shown that rapid adjustments reduce the effective radiative forcing (ERF) of black carbon by half of the instantaneous forcing, but for CO2 forcing, rapid adjustments increase ERF. Competing tropospheric adjustments for CO2 forcing are individually significant but sum to zero, such that the ERF equals the stratospherically adjusted radiative forcing, but this is not true for other forcing agents. Additional experiments of increase in the solar constant and increase in CH4 are used to show that a key factor of the rapid adjustment for an individual climate driver is changes in temperature in the upper troposphere and lower stratosphere.
Plain Language Summary
Long‐term global warming can be estimated with knowledge of how climate forcing agents affect the Earth's top‐of‐atmosphere energy imbalance or effective radiative forcing. Changes in climate forcers, such as greenhouse gases, the Sun's intensity, or emission of aerosol particles, typically impose a direct change in the energy budget, termed an instantaneous radiative forcing. Further to this, a climate forcer may induce changes in the atmosphere, such as a change in thermal structure, clouds, or humidity. These changes themselves, termed rapid adjustments, contribute to the top‐of‐atmosphere energy budget. Together, the instantaneous radiative forcing plus rapid adjustments equals the effective radiative forcing. We show that for different climate forcing agents, the rapid adjustments behave very differently and are driven by different atmospheric mechanisms. For example, rapid adjustments add to the instantaneous forcing for a carbon dioxide increase, due to a cooling of the stratosphere, but oppose instantaneous forcing for black carbon, driven by a warming troposphere and lowering of cloud height. Understanding rapid adjustments gives a more complete picture of the climate effects of different climate forcers.
Key Points
Rapid adjustments affect the Earth's energy balance in different ways for greenhouse gas, aerosol, and solar forcing
Radiative kernels and partial radiative perturbations are used to diagnose rapid adjustments from atmospheric and cloud changes
Noncloud adjustments agree well between models, whereas cloud adjustments exhibit more spread
Drivers of Precipitation Change Richardson, T. B.; Forster, P. M.; Andrews, T. ...
Journal of climate,
12/2018, Letnik:
31, Številka:
23
Journal Article
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The response of the hydrological cycle to climate forcings can be understood within the atmospheric energy budget framework. In this study precipitation and energy budget responses to five forcing ...agents are analyzed using 10 climate models from the Precipitation Driver Response Model Intercomparison Project (PDRMIP). Precipitation changes are split into a forcing-dependent fast response and a temperature-driven hydrological sensitivity. Globally, when normalized by top-of-atmosphere (TOA) forcing, fast precipitation changes are most sensitive to strongly absorbing drivers (CO2, black carbon). However, over land fast precipitation changes are most sensitive to weakly absorbing drivers (sulfate, solar) and are linked to rapid circulation changes. Despite this, land-mean fast responses to CO2 and black carbon exhibit more intermodel spread. Globally, the hydrological sensitivity is consistent across forcings, mainly associated with increased longwave cooling, which is highly correlated with intermodel spread. The land-mean hydrological sensitivity is weaker, consistent with limited moisture availability. The PDRMIP results are used to construct a simple model for land-mean and sea-mean precipitation change based on sea surface temperature change and TOA forcing. The model matches well with CMIP5 ensemble mean historical and future projections, and is used to understand the contributions of different drivers. During the twentieth century, temperature-driven intensification of land-mean precipitation has been masked by fast precipitation responses to anthropogenic sulfate and volcanic forcing, consistent with the small observed trend. However, as projected sulfate forcing decreases, and warming continues, land-mean precipitation is expected to increase more rapidly, and may become clearly observable by the mid-twenty-first century.
Precipitation is expected to respond differently to various drivers of anthropogenic climate change. We present the first results from the Precipitation Driver and Response Model Intercomparison ...Project (PDRMIP), where nine global climate models have perturbed CO2, CH4, black carbon, sulfate, and solar insolation. We divide the resulting changes to global mean and regional precipitation into fast responses that scale with changes in atmospheric absorption and slow responses scaling with surface temperature change. While the overall features are broadly similar between models, we find significant regional intermodel variability, especially over land. Black carbon stands out as a component that may cause significant model diversity in predicted precipitation change. Processes linked to atmospheric absorption are less consistently modeled than those linked to top‐of‐atmosphere radiative forcing. We identify a number of land regions where the model ensemble consistently predicts that fast precipitation responses to climate perturbations dominate over the slow, temperature‐driven responses.
Key Points
Precipitation response from five climate drivers shown for nine climate models
Fast responses scale with atmospheric absorption, slow with surface temperature
Over some land regions, fast precipitation responses dominate the slow response
Globally, latent heating associated with a change in precipitation is balanced by changes to atmospheric radiative cooling and sensible heat fluxes. Both components can be altered by climate forcing ...mechanisms and through climate feedbacks, but the impacts of climate forcing and feedbacks on sensible heat fluxes have received much less attention. Here we show, using a range of climate modelling results, that changes in sensible heat are the dominant contributor to the present global-mean precipitation change since preindustrial time, because the radiative impact of forcings and feedbacks approximately compensate. The model results show a dissimilar influence on sensible heat and precipitation from various drivers of climate change. Due to its strong atmospheric absorption, black carbon is found to influence the sensible heat very differently compared to other aerosols and greenhouse gases. Our results indicate that this is likely caused by differences in the impact on the lower tropospheric stability.
PDRMIP Myhre, G.; Forster, P. M.; Samset, B. H. ...
Bulletin of the American Meteorological Society,
06/2017, Letnik:
98, Številka:
6
Journal Article
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As the global temperature increases with changing climate, precipitation rates and patterns are affected through a wide range of physical mechanisms. The globally averaged intensity of extreme ...precipitation also changes more rapidly than the globally averaged precipitation rate. While some aspects of the regional variation in precipitation predicted by climate models appear robust, there is still a large degree of intermodel differences unaccounted for. Individual drivers of climate change initially alter the energy budget of the atmosphere, leading to distinct rapid adjustments involving changes in precipitation. Differences in how these rapid adjustment processes manifest themselves within models are likely to explain a large fraction of the present model spread and better quantifications are needed to improve precipitation predictions. Here, the authors introduce the Precipitation Driver and Response Model Intercomparison Project (PDRMIP), where a set of idealized experiments designed to understand the role of different climate forcing mechanisms were performed by a large set of climate models. PDRMIP focuses on understanding how precipitation changes relating to rapid adjustments and slower responses to climate forcings are represented across models. Initial results show that rapid adjustments account for large regional differences in hydrological sensitivity across multiple drivers. The PDRMIP results are expected to dramatically improve understanding of the causes of the present diversity in future climate projections.
Future projections of east Amazonian precipitation indicate drying, but they are uncertain and poorly understood. In this study we analyze the Amazonian precipitation response to individual ...atmospheric forcings using a number of global climate models. Black carbon is found to drive reduced precipitation over the Amazon due to temperature‐driven circulation changes, but the magnitude is uncertain. CO2 drives reductions in precipitation concentrated in the east, mainly due to a robustly negative, but highly variable in magnitude, fast response. We find that the physiological effect of CO2 on plant stomata is the dominant driver of the fast response due to reduced latent heating and also contributes to the large model spread. Using a simple model, we show that CO2 physiological effects dominate future multimodel mean precipitation projections over the Amazon. However, in individual models temperature‐driven changes can be large, but due to little agreement, they largely cancel out in the model mean.
Plain Language Summary
Climate models show that rainfall in the eastern Amazon may decrease during the 21st century; however, the changes are uncertain and there are many factors which could affect rainfall in the region. In this study we use a range of global climate model experiments to investigate how Amazonian rainfall responds to different drivers, such as carbon dioxide in the atmosphere. We find that increasing carbon dioxide reduces east Amazonian rainfall, and this is due to the response of plant stomata to carbon dioxide. Plant stomata do not open as wide when carbon dioxide is increased, which is known as the physiological effect. The physiological effect reduces evaporation from plants which means that there is less moisture available to fuel rainfall. We construct a simple model to estimate future rainfall changes over the Amazon to help fully understand the importance of physiological effects. The simple model shows that the physiological effect of carbon dioxide is the main driver of future drying over the eastern Amazon. This implies that future changes in rainfall are independent of how much the climate warms. Our findings show the importance of improving understanding of how plants affect atmospheric processes.
Key Points
Increased carbon dioxide consistently drives reduced eastern and central Amazonian precipitation in global climate models
Projected Amazonian precipitation changes are dominated by the carbon dioxide physiological effect
Highlights importance of reducing uncertainties associated with vegetation schemes
Different climate drivers influence precipitation in different ways. Here we use radiative kernels to understand the influence of rapid adjustment processes on precipitation in climate models. Rapid ...adjustments are generally triggered by the initial heating or cooling of the atmosphere from an external climate driver. For precipitation changes, rapid adjustments due to changes in temperature, water vapor, and clouds are most important. In this study we have investigated five climate drivers (CO2, CH4, solar irradiance, black carbon, and sulfate aerosols). The fast precipitation responses to a doubling of CO2 and a 10‐fold increase in black carbon are found to be similar, despite very different instantaneous changes in the radiative cooling, individual rapid adjustments, and sensible heating. The model diversity in rapid adjustments is smaller for the experiment involving an increase in the solar irradiance compared to the other climate driver perturbations, and this is also seen in the precipitation changes.
Plain Language Summary
Future projections of precipitation changes are uncertain, both on regional and global scales. Understanding the climate models' diversity of precipitation change and how these models respond to various climate drivers, such as greenhouse gases and aerosols, is a key topic in climate research. Using sophisticated techniques, we quantify the processes altering precipitation changes on a short time scale and show that changes in the vertical profile of temperature, water vapor, and clouds contribute very differently to precipitation changes for various climate drivers. Our results show that model diversity in precipitation changes varies strongly between the climate drivers.
Key Points
Separation of instantaneous and rapid adjustment contributions to precipitation changes
Contributions of rapid adjustments to precipitation changes differ substantially between climate drivers
Radiative kernels are applied to understand individual rapid adjustment terms
Efficacy of Climate Forcings in PDRMIP Models Richardson, T B; Forster, P M; Smith, C J ...
Journal of geophysical research. Atmospheres,
16 December 2019, Letnik:
124, Številka:
23
Journal Article
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Quantifying the efficacy of different climate forcings is important for understanding the real‐world climate sensitivity. This study presents a systematic multimodel analysis of different climate ...driver efficacies using simulations from the Precipitation Driver and Response Model Intercomparison Project (PDRMIP). Efficacies calculated from instantaneous radiative forcing deviate considerably from unity across forcing agents and models. Effective radiative forcing (ERF) is a better predictor of global mean near‐surface air temperature (GSAT) change. Efficacies are closest to one when ERF is computed using fixed sea surface temperature experiments and adjusted for land surface temperature changes using radiative kernels. Multimodel mean efficacies based on ERF are close to one for global perturbations of methane, sulfate, black carbon, and insolation, but there is notable intermodel spread. We do not find robust evidence that the geographic location of sulfate aerosol affects its efficacy. GSAT is found to respond more slowly to aerosol forcing than CO2 in the early stages of simulations. Despite these differences, we find that there is no evidence for an efficacy effect on historical GSAT trend estimates based on simulations with an impulse response model, nor on the resulting estimates of climate sensitivity derived from the historical period. However, the considerable intermodel spread in the computed efficacies means that we cannot rule out an efficacy‐induced bias of ±0.4 K in equilibrium climate sensitivity to CO2 doubling when estimated using the historical GSAT trend.
We present the global and regional hydrological sensitivity (HS) to surface temperature changes, for perturbations to CO2, CH4, sulfate and black carbon concentrations, and solar irradiance. Based on ...results from ten climate models, we show how modeled global mean precipitation increases by 2-3% per kelvin of global mean surface warming, independent of driver, when the effects of rapid adjustments are removed. Previously reported differences in response between drivers are therefore mainly ascribable to rapid atmospheric adjustment processes. All models show a sharp contrast in behavior over land and over ocean, with a strong surface temperature-driven (slow) ocean HS of 3-5%/K, while the slow land HS is only 0-2%/K. Separating the response into convective and large-scale cloud processes, we find larger inter-model differences, in particular over land regions. Large-scale precipitation changes are most relevant at high latitudes, while the equatorial HS is dominated by convective precipitation changes. Black carbon stands out as the driver with the largest inter-model slow HS variability, and also the strongest contrast between a weak land and strong sea response. We identify a particular need for model investigations and observational constraints on convective precipitation in the Arctic, and large-scale precipitation around the Equator.