Global Mean Precipitation Changes Research Articles

The Choice of Baseline Period Influences the Assessments of the Outcomes of Stratospheric Aerosol Injection

AbstractThe specifics of the simulated injection choices in the case of stratospheric aerosol injections (SAI) are part of the crucial context necessary for meaningfully discussing the impacts that a deployment of SAI would have on the planet. One of the main choices is the desired amount of cooling that the injections are aiming to achieve. Previous SAI simulations have usually either simulated a fixed amount of injection, resulting in a fixed amount of warming being offset, or have specified one target temperature, so that the amount of cooling is only dependent on the underlying trajectory of greenhouse gases. Here, we use three sets of SAI simulations achieving different amounts of global mean surface cooling while following a middle‐of‐the‐road greenhouse gas emission trajectory: one SAI scenario maintains temperatures at 1.5°C above preindustrial levels (PI), and two other scenarios which achieve additional cooling to 1.0°C and 0.5°C above PI. We demonstrate that various surface impacts scale proportionally with respect to the amount of cooling, such as global mean precipitation changes, changes to the Atlantic Meridional Overturning Circulation and to the Walker Cell. We also highlight the importance of the choice of the baseline period when comparing the SAI responses to one another and to the greenhouse gas emission pathway. This analysis leads to policy‐relevant discussions around the concept of a reference period altogether, and to what constitutes a relevant, or significant, change produced by SAI.

Observed variability and trends in global precipitation during 1979–2020

How global precipitation might have changed on the interdecadal-to-multi-decadal time scales during the satellite (post-1979) era is examined by means of the satellite-based GPCP V2.3 monthly precipitation analysis. Comparisons with the results from CMIP6 and AMIP6 are further made in terms of global mean precipitation change and regional features of precipitation change, aiming to provide not only an improved understanding of the effects of major physical mechanisms on precipitation change, but also an assessment of the skills of current climate models and likely some clues for diagnosing possible limitations in observed precipitation. Long-term change/trend in global mean precipitation is generally weak in GPCP. Although the GPCP trend is statistically significant at the 90% confidence level over global land + ocean during 1979–2020, it is not significant over either global land or ocean separately. For the shorter, overlap period with the CMIP6 historical experiments (1979–2014), GPCP positive trends can’t reach the 90% confidence level, while significant and more intense precipitation trends appear in CMIP6 ensemble-means. However, a roughly similar global sensitivity to surface temperature change can be derived in GPCP, CMIP6, and AMIP6, providing confidence in both observed and simulated global mean precipitation change. Large regional trends with positive and negative values can readily be seen across the world in GPCP. AMIP6 can generally reproduce these large-scale spatial features. Comparisons with CMIP6 confirm the combined effects from anthropogenic greenhouse-gases (GHG) forcing and internal modes of climate variability such as the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO). Limiting the PDO/AMO effect makes the trend patterns in GPCP residuals more similar to those in CMIP6, implying that the GHG effect would become more readily detectable in observed precipitation in the near future with regards to both global mean and regional precipitation changes. Furthermore, similar changes in precipitation seasonal range, especially over global lands, occur in GPCP, CMIP6, and AMIP6, suggesting that the GHG effect might already be discernible in certain aspects of precipitation change.

On the contribution of fast and slow responses to precipitation changes caused by aerosol perturbations

Abstract. Changes in global-mean precipitation are strongly constrained by global radiative cooling, while regional rainfall changes are less constrained because energy can be transported. Absorbing and non-absorbing aerosols have different effects on both global-mean and regional precipitation, due to the distinct effects on energetics. This study analyses the precipitation responses to large perturbations in black carbon (BC) and sulfate (SUL) by examining the changes in atmospheric energy budget terms on global and regional scales, in terms of fast (independent of changes in sea surface temperature, SST) and slow responses (mediated by changes in SST). Changes in atmospheric radiative cooling/heating are further decomposed into contributions from clouds, aerosols, and clear–clean sky (without clouds or aerosols). Both cases show a decrease in global-mean precipitation, which is dominated by fast responses in the BC case and slow responses in the SUL case. The geographical patterns are distinct too. The intertropical convergence zone (ITCZ), accompanied by tropical rainfall, shifts northward in the BC case, while it shifts southward in the SUL case. For both cases, energy transport terms from the slow response dominate the changes in tropical rainfall, which are associated with the northward (southward) shift of the Hadley cell in response to the enhanced southward (northward) cross-equatorial energy flux caused by increased BC (SUL) emission. The extra-tropical precipitation decreases in both cases. For the BC case, fast responses to increased atmospheric radiative heating contribute most to the reduced rainfall, in which absorbing aerosols directly heat the mid-troposphere, stabilise the column, and suppress precipitation. Unlike BC, non-absorbing aerosols decrease surface temperatures through slow processes, cool the whole atmospheric column, and reduce specific humidity, which leads to decreased radiative cooling from the clear–clean sky, which is consistent with the reduced rainfall. Examining the changes in large-scale circulation and local thermodynamics qualitatively explains the responses of precipitation to aerosol perturbations, whereas the energetic perspective provides a method to quantify their contributions.

On the Contribution of Fast and Slow Responses to Precipitation Changes Caused by Aerosol Perturbations

Abstract. Changes in global-mean precipitation are strongly constrained by global radiative cooling, while regional rainfall changes are less constrained because energy can be transported. Absorbing and non-absorbing aerosols have different effects on both global-mean and regional precipitation, due to the distinct effects on energetics. This study analyses the precipitation responses to large perturbations in black carbon (BC) and sulphate (SUL) respectively by examining the changes in atmospheric energy budget terms on global and regional scales, in terms of fast (independent of changes in sea surface temperature (SST)) and slow responses (mediated by changes in SST). Changes in atmospheric radiative cooling/heating are further decomposed into contributions from clouds, aerosols, and clear-clean sky (without clouds or aerosols). Both cases show a decrease in global-mean precipitation, dominated by fast responses in the BC case while slow responses in the SUL case. The geographical patterns are distinct too. The intertropical convergence zone (ITCZ), accompanied with tropical rainfall, shifts northward in the BC case, while southward in the SUL case. For both cases, energy transport terms from the slow response dominates the changes in tropical rainfall, which are associated with the northward (southward) shift of Hadley cell in response to the enhanced southward (northward) cross-equatorial energy flux caused by increased BC (SUL) emission. The extra-tropical precipitation decreases in both cases. For the BC case, fast responses to increased atmospheric radiative heating contribute most to the reduced rainfall, in which absorbing aerosols directly heat the mid-troposphere, stabilise the column, and suppress precipitation. Unlike BC, non-absorbing aerosols decrease surface temperatures through slow processes, cool the whole atmospheric column, and reduce specific humidity, which leads to decreased radiative cooling from the clean-clear sky, and is consistent with the reduced rainfall. Examining the changes in large-scale circulation and local thermodynamics qualitatively explains the responses of precipitation to aerosol perturbations, whereas the energetic perspective provides a method to quantify their contributions.

Significant climate impacts of aerosol changes driven by growth in energy use and advances in emission control technology

Abstract. Anthropogenic aerosols have increased significantly since the industrial revolution, driven largely by growth in emissions from energy use in sectors including power generation, industry, and transport. Advances in emission control technologies since around 1970, however, have partially counteracted emissions increases from the above sectors. Using the fully coupled Community Earth System Model, we quantify the effective radiative forcing (ERF) and climate response to 1970–2010 aerosol changes associated with the above two policy-relevant emission drivers. Emissions from energy-use growth generate a global mean aerosol ERF (mean ± 1 standard deviation) of -0.31±0.22 W m−2 and result in a global mean cooling (-0.35±0.17 K) and a precipitation reduction (-0.03±0.02 mm d−1). By contrast, the avoided emissions from advances in emission control technology, which benefit air quality, generate a global mean ERF of +0.21±0.23 W m−2, a global warming of +0.10±0.13 K, and global mean precipitation increase of +0.01±0.02 mm d−1. Despite the relatively small changes in global mean precipitation, these two emission drivers have profound impacts at regional scales, in particular over Asia and Europe. The total net aerosol impacts on climate are dominated by energy-use growth, from Asia in particular. However, technology advances outweigh energy-use growth over Europe and North America. Various non-linear processes are involved along the pathway from aerosol and their precursor emissions to radiative forcing and ultimately to climate responses, suggesting that the diagnosed aerosol forcing and effects must be interpreted in the context of experiment designs. Further, the temperature response per unit aerosol ERF varies significantly across many factors, including location and magnitude of emission changes, implying that ERF, and the related metrics, needs to be used very carefully for aerosols. Future aerosol-related emission pathways have large temporal and spatial uncertainties; our findings provide useful information for both assessing and interpreting such uncertainties, and they may help inform future climate change impact reduction strategies.

Analysis of the Atmospheric Water Budget for Elucidating the Spatial Scale of Precipitation Changes Under Climate Change.

Global mean precipitation changes due to climate change were previously shown to be relatively small and well constrained by the energy budget. However, local precipitation changes can be much more significant. In this paper we propose that for large enough scales, for which the water budget is closed (precipitation [P] roughly equals evaporation [E]), changes in P approach the small global mean value. However, for smaller scales, for which P and E are not necessarily equal and convergence of water vapor still plays a role, changes in P could be much larger due to dynamical contributions. Using 40 years of two reanalysis data sets, 39 Coupled Model Intercomparison Project Phase 5 (CMIP5) models and additional numerical simulations, we identify the scale of transition in the importance of the different terms in the water budget to precipitation to be ~3,500–4,000 km and demonstrate its relation to the spatial scale of precipitation changes under climate change.

On the influence of simulated SST warming on rainfall projections in the Indo-Pacific domain: an AGCM study

Significant uncertainty exists in regional climate change projections, particularly for rainfall and other hydro-climate variables. In this study, we conduct a series of Atmospheric General Circulation Model (AGCM) experiments with different future sea surface temperature (SST) warming simulated by a range of coupled climate models. They allow us to assess the extent to which uncertainty from current coupled climate model rainfall projections can be attributed to their simulated SST warming. Nine CMIP5 model-simulated global SST warming anomalies have been super-imposed onto the current SSTs simulated by the Australian climate model ACCESS1.3. The ACCESS1.3 SST-forced experiments closely reproduce rainfall means and interannual variations as in its own fully coupled experiments. Although different global SST warming intensities explain well the inter-model difference in global mean precipitation changes, at regional scales the SST influence vary significantly. SST warming explains about 20–25% of the patterns of precipitation changes in each of the four/five models in its rainfall projections over the oceans in the Indo-Pacific domain, but there are also a couple of models in which different SST warming explains little of their precipitation pattern changes. The influence is weaker again for rainfall changes over land. Roughly similar levels of contribution can be attributed to different atmospheric responses to SST warming in these models. The weak SST influence in our study could be due to the experimental setup applied: superimposing different SST warming anomalies onto the same SSTs simulated for current climate by ACCESS1.3 rather than directly using model-simulated past and future SSTs. Similar modelling and analysis from other modelling groups with more carefully designed experiments are needed to tease out uncertainties caused by different SST warming patterns, different SST mean biases and different model physical/dynamical responses to the same underlying SST forcing.

Inferring Aerosol Cooling from Hydrological Sensitivity

Abstract Hydrological sensitivity is the change in global-mean precipitation per degree of global-mean temperature change. This paper shows that the hydrological sensitivity of the response to anthropogenic aerosol forcing is distinct from that of the combined response to all other forcings and that this difference is sufficient to infer the associated cooling in global-mean temperature. This result is demonstrated using temperature and precipitation data generated by climate models and is robust across different climate models. Remarkably, greenhouse gas warming and aerosol cooling can be estimated in a model without using any spatial or temporal gradient information in the response, provided temperature data are augmented by precipitation data. Over the late twentieth century, the hydrological sensitivities of climate models differ significantly from that of observations. Whether this discrepancy can be attributed to observational error, which is substantial as different estimates of global-mean precipitation are not even significantly correlated with each other, or to model error is unclear. The results highlight the urgency to construct accurate estimates of global precipitation from past observations and for reducing model uncertainty in hydrological sensitivity. This paper also clarifies that previous estimates of hydrological sensitivity are limited in that standard regression methods neglect temperature–precipitation relations that occur through internal variability. An alternative method for estimating hydrological sensitivity that overcomes this limitation is presented.

An intensified hydrological cycle in the simulation of geoengineering by cirrus cloud thinning using ice crystal fall speed changes

AbstractProposals to geoengineer Earth's climate by cirrus cloud thinning (CCT) potentially offer advantages over solar radiation management schemes: amplified cooling of the Arctic and smaller perturbations to global mean precipitation in particular. Using an idealized climate model implementation of CCT in which ice particle fall speeds were increased 2×, 4×, and 8× we examine the relationships between effective radiative forcing (ERF) at the top of atmosphere, near‐surface temperature, and the response of the hydrological cycle. ERF was nonlinear with fall speed change and driven by the trade‐off between opposing positive shortwave and negative longwave radiative forcings. ERF was −2.0 Wm−2 for both 4× and 8× fall speeds. Global mean temperature decreased linearly with ERF, while Arctic temperature reductions were amplified compared with the global mean change. The change in global mean precipitation involved a rapid adjustment (~ 1%/Wm2), which was linear with the change in the net atmospheric energy balance, and a feedback response (~2%/°C). Global mean precipitation and evaporation increased strongly in the first year of CCT. Intensification of the hydrological cycle was promoted by intensification of the vertical overturning circulation of the atmosphere, changes in boundary layer climate favorable for evaporation, and increased energy available at the surface for evaporation (from increased net shortwave radiation and reduced subsurface storage of heat). Such intensification of the hydrological cycle is a significant side effect to the cooling of climate by CCT. Any accompanying negative cirrus cloud feedback response would implicitly increase the costs and complexity of CCT deployment.

Radiative and climate impacts of a large volcanic eruption during stratospheric sulfur geoengineering

Abstract. Both explosive volcanic eruptions, which emit sulfur dioxide into the stratosphere, and stratospheric geoengineering via sulfur injections can potentially cool the climate by increasing the amount of scattering particles in the atmosphere. Here we employ a global aerosol-climate model and an Earth system model to study the radiative and climate changes occurring after an erupting volcano during solar radiation management (SRM). According to our simulations the radiative impacts of the eruption and SRM are not additive and the radiative effects and climate changes occurring after the eruption depend strongly on whether SRM is continued or suspended after the eruption. In the former case, the peak burden of the additional stratospheric sulfate as well as changes in global mean precipitation are fairly similar regardless of whether the eruption takes place in a SRM or non-SRM world. However, the maximum increase in the global mean radiative forcing caused by the eruption is approximately 21 % lower compared to a case when the eruption occurs in an unperturbed atmosphere. In addition, the recovery of the stratospheric sulfur burden and radiative forcing is significantly faster after the eruption, because the eruption during the SRM leads to a smaller number and larger sulfate particles compared to the eruption in a non-SRM world. On the other hand, if SRM is suspended immediately after the eruption, the peak increase in global forcing caused by the eruption is about 32 % lower compared to a corresponding eruption into a clean background atmosphere. In this simulation, only about one-third of the global ensemble-mean cooling occurs after the eruption, compared to that occurring after an eruption under unperturbed atmospheric conditions. Furthermore, the global cooling signal is seen only for the 12 months after the eruption in the former scenario compared to over 40 months in the latter. In terms of global precipitation rate, we obtain a 36 % smaller decrease in the first year after the eruption and again a clearly faster recovery in the concurrent eruption and SRM scenario, which is suspended after the eruption. We also found that an explosive eruption could lead to significantly different regional climate responses depending on whether it takes place during geoengineering or into an unperturbed background atmosphere. Our results imply that observations from previous large eruptions, such as Mount Pinatubo in 1991, are not directly applicable when estimating the potential consequences of a volcanic eruption during stratospheric geoengineering.

The cloud radiative effect on the atmospheric energy budget and global mean precipitation

This study seeks to explain the effects of cloud on changes in atmospheric radiative absorption that largely balance changes in global mean precipitation under climate change. The partial radiative perturbations (PRPs) due to changes in cloud and due to the effects of the pre-existing climatological cloud distribution on non-cloud changes, known as “cloud masking”, are calculated when atmospheric CO2 concentration is doubled for the HadSM3 and MIROC models and for a large ensemble of parameter perturbed models based on HadSM3. Because the effect of cloud on changes in atmospheric shortwave absorption is almost negligible, longwave fluxes are analysed alone. We find that the net effects of cloud masking and cloud PRP on atmospheric absorption are both substantial. For the tropics, our results are reviewed in light of hypotheses put forward to explain cloud and radiative flux changes. We find that the major effects of clouds on radiation change are linked to known physical processes that are quite consistently simulated by models. Cloud top height changes are quite well described by the fixed anvil temperature hypothesis of Hartmann and Larson; cloud base heights change little, remaining near the same pressure. Changes in cloud geographical location and cloud amount are significant, but play a smaller role in driving radiative flux changes. Finally, because clouds are a large source of modelling uncertainty, we consider whether resolving errors in cloud simulation could reconcile modelled global mean precipitation trends of about 1–3 %\(\hbox {K}^{-1}\) with some estimates of observed trends of 7 %\(\hbox {K}^{-1}\) or more. This would require the radiative effect of clouds to change from one that increases atmospheric radiative absorption by about \(0.5\,\hbox {Wm}^{-2}\,\hbox {K}^{-1}\) to one that decreases it by \(-3.5\,\hbox {Wm}^{-2}\,\hbox {K}^{-1}\). Based on our results, this seems difficult to achieve within our current rationale for the tropics at least.

Projected effects of declining aerosols in RCP4.5: unmasking global warming?

Abstract. All the representative concentration pathways (RCPs) include declining aerosol emissions during the 21st century, but the effects of these declines on climate projections have had little attention. Here we assess the global and hemispheric-scale effects of declining anthropogenic aerosols in RCP4.5 in CSIRO-Mk3.6, a model from the Coupled Model Intercomparison Project Phase 5 (CMIP5). Results from this model are then compared with those from other CMIP5 models. We calculate the aerosol effective radiative forcing (ERF, including indirect effects) in CSIRO-Mk3.6 relative to 1850, using a series of atmospheric simulations with prescribed sea-surface temperatures (SST). Global-mean aerosol ERF at the top of the atmosphere is most negative in 2005 (−1.47 W m−2). Between 2005 and 2100 it increases by 1.46 W m−2, i.e., it approximately returns to 1850 levels. Although increasing greenhouse gases (GHGs) and declining aerosols both exert a positive ERF at the top of the atmosphere during the 21st century, they have opposing effects on radiative heating of the atmosphere: increasing GHGs warm the atmosphere, whereas declining aerosols cool the atmosphere due to reduced absorption of shortwave radiation by black carbon (BC). We then compare two projections for 2006–2100, using the coupled atmosphere-ocean version of the model. One (RCP45) follows the usual RCP4.5; the other (RCP45A2005) has identical forcing, except that emissions of anthropogenic aerosols and precursors are fixed at 2005 levels. The global-mean surface warming in RCP45 is 2.3 °C per 95 yr, of which almost half (1.1 °C) is caused by declining aerosols. The warming due to declining aerosols is almost twice as strong in the Northern Hemisphere as in the Southern Hemisphere, whereas that due to increasing GHGs is similar in the two hemispheres. For precipitation changes, the effects of declining aerosols are larger than those of increasing GHGs due to decreasing atmospheric absorption by black carbon: 63% of the projected global-mean precipitation increase of 0.16 mm per day is caused by declining aerosols. In the Northern Hemisphere, precipitation increases by 0.29 mm per day, of which 72% is caused by declining aerosols. Comparing 13 CMIP5 models, we find a correlation of –0.54 (significant at 5%) between aerosol ERF in the present climate and projected global-mean surface warming in RCP4.5; thus, models that have more negative aerosol ERF in the present climate tend to project stronger warming during 2006–2100. A similar correlation (–0.56) is found between aerosol ERF and projected changes in global-mean precipitation. These results suggest that aerosol forcing substantially modulates projected climate response in RCP4.5. In some respects, the effects of declining aerosols are quite distinct from those of increasing GHGs. Systematic efforts are needed to better quantify the role of declining aerosols in climate projections.

Global Hydrological Cycle Response to Rapid and Slow Global Warming

AbstractThis study analyzes the response of global water vapor to global warming in a series of fully coupled climate model simulations. The authors find that a roughly 7% K−1 rate of increase of water vapor with global surface temperature is robust only for rapid anthropogenic-like climate change. For slower warming that occurred naturally in the past, the Southern Ocean has time to equilibrate, producing a different pattern of surface warming, so that water vapor increases at only 4.2% K−1. This lower rate of increase of water vapor with warming is not due to relative humidity changes or differences in mean lower-tropospheric temperature. A temperature of over 80°C would be required in the Clausius–Clapeyron relationship to match the 4.2% K−1 rate of increase. Instead, the low rate of increase is due to spatially heterogeneous warming. During slower global warming, there is enhanced warming at southern high latitudes, and hence less warming in the tropics per kelvin of global surface temperature increase. This leads to a smaller global water vapor increase, because most of the atmospheric water vapor is in the tropics. A formula is proposed that applies to general warming scenarios. This study also examines the response of global-mean precipitation and the meridional profile of precipitation minus evaporation and compares the latter to thermodynamic scalings. It is found that global-mean precipitation changes are remarkably robust between rapid and slow warming. Thermodynamic scalings for the rapid- and slow-warming zonal-mean precipitation are similar, but the precipitation changes are significantly different, suggesting that circulation changes are important in driving these differences.

Hydrological sensitivity to greenhouse gases and aerosols in a global climate model

Changes in greenhouse gases and aerosols alter the atmospheric energy budget on different time scales and at different levels in the atmosphere. We study the relationship between global mean precipitation changes, radiative forcing, and surface temperature change since preindustrial times caused by several climate change components (CO2, CH4, sulphate and black carbon (BC) aerosols, and solar forcing) using the National Center for Atmospheric Research Community Earth System Model (CESM1.03). We find a fast response in precipitation due to atmospheric instability that correlates with radiative forcing associated with atmospheric absorption and a slower response caused by changes in surface temperature which correlates with radiative forcing at the top of the atmosphere. In general, global climate models show large differences in climate response to global warming, but here we find a strong relationship between global mean radiative forcing and global mean precipitation changes that is very consistent with other models, indicating that precipitation changes from a particular forcing mechanism are more robust than previously expected. In addition, we look at the precipitation response and relate it to changes in lifetime of atmospheric water vapor (τ). BC aerosols have a significantly larger impact on changes in τ related to surface temperature compared to greenhouse gases, sulphate aerosols, and solar forcing and are the dominating forcing mechanism affecting fast precipitation in this quantity.

Climate change under aggressive mitigation: the ENSEMBLES multi-model experiment

We present results from multiple comprehensive models used to simulate an aggressive mitigation scenario based on detailed results of an Integrated Assessment Model. The experiment employs ten global climate and Earth System models (GCMs and ESMs) and pioneers elements of the long-term experimental design for the forthcoming 5th Intergovernmental Panel on Climate Change assessment. Atmospheric carbon-dioxide concentrations pathways rather than carbon emissions are specified in all models, including five ESMs that contain interactive carbon cycles. Specified forcings also include minor greenhouse gas concentration pathways, ozone concentration, aerosols (via concentrations or precursor emissions) and land use change (in five models). The new aggressive mitigation scenario (E1), constructed using an integrated assessment model (IMAGE 2.4) with reduced fossil fuel use for energy production aimed at stabilizing global warming below 2 K, is studied alongside the medium-high non-mitigation scenario SRES A1B. Resulting twenty-first century global mean warming and precipitation changes for A1B are broadly consistent with previous studies. In E1 twenty-first century global warming remains below 2 K in most models, but global mean precipitation changes are higher than in A1B up to 2065 and consistently higher per degree of warming. The spread in global temperature and precipitation responses is partly attributable to inter-model variations in aerosol loading and representations of aerosol-related radiative forcing effects. Our study illustrates that the benefits of mitigation will not be realised in temperature terms until several decades after emissions reductions begin, and may vary considerably between regions. A subset of the models containing integrated carbon cycles agree that land and ocean sinks remove roughly half of present day anthropogenic carbon emissions from the atmosphere, and that anthropogenic carbon emissions must decrease by at least 50% by 2050 relative to 1990, with further large reductions needed beyond that to achieve the E1 concentrations pathway. Negative allowable anthropogenic carbon emissions at and beyond 2100 cannot be ruled out for the E1 scenario. There is self-consistency between the multi-model ensemble of allowable anthropogenic carbon emissions and the E1 scenario emissions from IMAGE 2.4.

Changes in global-mean precipitation in response to warming, greenhouse gas forcing and black carbon

Precipitation changes are a key driver of climate change impacts. On average, global precipitation is expected to increase with warming. However, model projections show that precipitation does not scale linearly with surface air temperature. Instead, global hydrological sensitivity, the relative change of global-mean precipitation per degree of global warming, seems to vary across different scenarios and even with time. Based on output from 20 coupled Atmosphere-Ocean-General-Circulation-Models for up to 7 different scenarios, we discuss to what extent these variations can be explained by changes in the tropospheric energy budget. Our analysis supports earlier findings that long-and shortwave absorbers initially decrease global-mean precipitation. Including these absorbers into a multivariate scaling approach allows to closely reproduce the simulated global-mean precipitation changes. We find a sensitivity of global-mean precipitation to tropospheric greenhouse gas forcing of -0.42 +/- 0.23%/(W/m(2)) ( uncertainty given as one std of inter-model variability) and to black carbon emissions of -0.07 +/- 0.02%/(Mt/yr). In combination with these two predictors the dominant longer-term effect of surface air temperatures on precipitation is estimated to be 2.2 +/- 0.52%/K - much lower than the 6.5%/K that may be expected from the Clausius-Clapeyron relationship. .

Fast versus slow response in climate change: implications for the global hydrological cycle

Recent studies have shown that changes in global mean precipitation are larger for solar forcing than for CO2 forcing of similar magnitude.In this paper, we use an atmospheric general circulation model to show that the differences originate from differing fast responses of the climate system. We estimate the adjusted radiative forcing and fast response using Hansen's ``fixed-SST forcing'' method.Total climate system response is calculated using mixed layer simulations using the same model. Our analysis shows that the fast response is almost 40% of the total response for few key variables like precipitation and evaporation. We further demonstrate that the hydrologic sensitivity, defined as the change in global mean precipitation per unit warming, is the same for the two forcings when the fast responses are excluded from the definition of hydrologic sensitivity, suggesting that the slow response (feedback) of the hydrological cycle is independent of the forcing mechanism. Based on our results, we recommend that the fast and slow response be compared separately in multi-model intercomparisons to discover and understand robust responses in hydrologic cycle. The significance of this study to geoengineering is discussed.

Impact of geoengineering schemes on the global hydrological cycle

The rapidly rising CO(2) level in the atmosphere has led to proposals of climate stabilization by "geoengineering" schemes that would mitigate climate change by intentionally reducing solar radiation incident on Earth's surface. In this article we address the impact of these climate stabilization schemes on the global hydrological cycle. By using equilibrium climate simulations, we show that insolation reductions sufficient to offset global-scale temperature increases lead to a decrease in global mean precipitation. This occurs because solar forcing is more effective in driving changes in global mean evaporation than is CO(2) forcing of a similar magnitude. In the model used here, the hydrological sensitivity, defined as the percentage change in global mean precipitation per degree warming, is 2.4% K(-1) for solar forcing, but only 1.5% K(-1) for CO(2) forcing. Although other models and the climate system itself may differ quantitatively from this result, the conclusion can be understood based on simple considerations of the surface energy budget and thus is likely to be robust. For the same surface temperature change, insolation changes result in relatively larger changes in net radiative fluxes at the surface; these are compensated by larger changes in the sum of latent and sensible heat fluxes. Hence, the hydrological cycle is more sensitive to temperature adjustment by changes in insolation than by changes in greenhouse gases. This implies that an alteration in solar forcing might offset temperature changes or hydrological changes from greenhouse warming, but could not cancel both at once.