16 research outputs found
Dynamical response of Mediterranean precipitation to greenhouse gases and aerosols
Atmospheric aerosols and greenhouse gases affect cloud properties, radiative balance and, thus, the hydrological cycle. Observations show that precipitation has decreased in the Mediterranean since the beginning of the 20th century, and many studies have investigated possible mechanisms. So far, however, the effects of aerosol forcing on Mediterranean precipitation remain largely unknown. Here we compare the modeled dynamical response of Mediterranean precipitation to individual forcing agents in a set of global climate models (GCMs). Our analyses show that both greenhouse gases and aerosols can cause drying in the Mediterranean and that precipitation is more sensitive to black carbon (BC) forcing than to well-mixed greenhouse gases (WMGHGs) or sulfate aerosol. In addition to local heating, BC appears to reduce precipitation by causing an enhanced positive sea level pressure (SLP) pattern similar to the North Atlantic Oscillation–Arctic Oscillation, characterized by higher SLP at midlatitudes and lower SLP at high latitudes. WMGHGs cause a similar SLP change, and both are associated with a northward diversion of the jet stream and storm tracks, reducing precipitation in the Mediterranean while increasing precipitation in northern Europe. Though the applied forcings were much larger, if forcings are scaled to those of the historical period of 1901–2010, roughly one-third (31±17%) of the precipitation decrease would be attributable to global BC forcing with the remainder largely attributable to WMGHGs, whereas global scattering sulfate aerosols would have negligible impacts. Aerosol–cloud interactions appear to have minimal impacts on Mediterranean precipitation in these models, at least in part because many simulations did not fully include such processes; these merit further study. The findings from this study suggest that future BC and WMGHG emissions may significantly affect regional water resources, agricultural practices, ecosystems and the economy in the Mediterranean region
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PDRMIP: a precipitation driver and response model intercomparison project - protocol and preliminary results
PDRMIP investigates the role of various drivers of climate change for mean and extreme precipitation changes, based on multiple climate model output and energy budget analyses.
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 inter-model 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 needs better quantifications to improve precipitation predictions. Here, we 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 our understanding of the causes of the present diversity in future climate projections
Marine cloud brightening – as effective without clouds
Marine cloud brightening through sea spray injection has been proposed as
a climate engineering method for avoiding the most severe consequences of
global warming. A limitation of most of the previous modelling studies on
marine cloud brightening is that they have either considered individual
models or only investigated the effects of a specific increase in the number
of cloud droplets. Here we present results from coordinated simulations with
three Earth system models (ESMs) participating in the Geoengineering Model
Intercomparison Project (GeoMIP) G4sea-salt experiment. Injection rates of
accumulation-mode sea spray aerosol particles over ocean between
30° N and 30° S are set in each model to generate
a global-mean effective radiative forcing (ERF) of −2.0 W m−2
at the top of the atmosphere. We find that the injection increases the cloud
droplet number concentration in lower layers, reduces the cloud-top effective
droplet radius, and increases the cloud optical depth over the injection
area. We also find, however, that the global-mean clear-sky ERF by the
injected particles is as large as the corresponding total ERF in all three
ESMs, indicating a large potential of the aerosol direct effect in regions of
low cloudiness. The largest enhancement in ERF due to the presence of clouds
occur as expected in the subtropical stratocumulus regions off the west
coasts of the American and African continents. However, outside these
regions, the ERF is in general equally large in cloudy and clear-sky
conditions. These findings suggest a more important role of the aerosol
direct effect in sea spray climate engineering than previously thought
Cosmic rays, cloud condensation nuclei and clouds – a reassessment using MODIS data
The response of clouds to sudden decreases in the flux of galactic cosmic rays (GCR) – Forbush decrease events – has been investigated using cloud products from the space-borne MODIS instrument, which has been in operation since 2000. By focusing on pristine Southern Hemisphere ocean regions we examine areas where we believe that a cosmic ray signal should be easier to detect than elsewhere. While previous studies have mainly considered cloud cover, the high spatial and spectral resolution of MODIS allows for a more thorough study of microphysical parameters such as cloud droplet size, cloud water content and cloud optical depth, in addition to cloud cover. Averaging the results from the 22 Forbush decrease events that were considered, no statistically significant correlations were found between any of the four cloud parameters and GCR, when autocorrelations were taken into account. Splitting the area of study into six domains, all of them have a negative correlation between GCR and cloud droplet size, in agreement with a cosmic ray – cloud coupling, but in only one of the domains (eastern Atlantic Ocean) was the correlation statistically significant. Conversely, cloud optical depth is mostly negatively correlated with GCR, and in the eastern Atlantic Ocean domain that correlation is statistically significant. For cloud cover and liquid water path, the correlations with GCR are weaker, with large variations between the different domains. When only the six Forbush decrease events with the largest amplitude (more than 10% decrease) were studied, the correlations fit the hypothesis slightly better, with 16 out of 24 correlations having the expected sign, although many of the correlations are quite weak. Introducing a time lag of a few days for clouds to respond to the cosmic ray signal the correlations tend to become weaker and even to change sign
Response to marine cloud brightening in a multi-model ensemble
Here we show results from Earth system model simulations from the
marine cloud brightening experiment G4cdnc of the Geoengineering Model
Intercomparison Project (GeoMIP). The nine contributing models prescribe a
50 % increase in the cloud droplet number concentration (CDNC) of low
clouds over the global oceans in an experiment dubbed G4cdnc, with the
purpose of counteracting the radiative forcing due to anthropogenic
greenhouse gases under the RCP4.5 scenario. The model ensemble median
effective radiative forcing (ERF) amounts to −1.9 W m−2, with a
substantial inter-model spread of −0.6 to −2.5 W m−2. The large spread
is partly related to the considerable differences in clouds and their
representation between the models, with an underestimation of low clouds in
several of the models. All models predict a statistically significant
temperature decrease with a median of (for years 2020–2069) −0.96 [−0.17 to
−1.21] K relative to the RCP4.5 scenario, with particularly strong cooling
over low-latitude continents. Globally averaged there is a weak but
significant precipitation decrease of −2.35 [−0.57 to −2.96] % due to a
colder climate, but at low latitudes there is a 1.19 % increase over
land. This increase is part of a circulation change where a strong negative
top-of-atmosphere (TOA) shortwave forcing over subtropical oceans, caused
by increased albedo associated with the increasing CDNC, is compensated for by
rising motion and positive TOA longwave signals over adjacent land regions
Global and regional radiative forcing from 20 % reductions in BC, OC and SO<sub>4</sub> – an HTAP2 multi-model study
In the Hemispheric Transport of Air Pollution Phase 2 (HTAP2) exercise, a
range of global atmospheric general circulation and chemical transport
models performed coordinated perturbation experiments with 20 %
reductions in emissions of anthropogenic aerosols, or aerosol precursors, in
a number of source regions. Here, we compare the resulting changes in the
atmospheric load and vertically resolved profiles of black carbon (BC),
organic aerosols (OA) and sulfate (SO4) from 10 models that include
treatment of aerosols. We use a set of temporally, horizontally and
vertically resolved profiles of aerosol forcing efficiency (AFE) to estimate
the impact of emission changes in six major source regions on global
radiative forcing (RF) pertaining to the direct aerosol effect, finding
values between. 51.9 and 210.8 mW m−2 Tg−1 for BC, between −2.4
and −17.9 mW m−2 Tg−1 for OA and between −3.6 and −10.3 W m−2 Tg−1 for SO4. In most cases, the local influence
dominates, but results show that mitigations in south and east Asia have
substantial impacts on the radiative budget in all investigated receptor
regions, especially for BC. In Russia and the Middle East, more than 80 %
of the forcing for BC and OA is due to extra-regional emission reductions.
Similarly, for North America, BC emissions control in east Asia is found to
be more important than domestic mitigations, which is consistent with
previous findings. Comparing fully resolved RF calculations to RF estimates
based on vertically averaged AFE profiles allows us to quantify the
importance of vertical resolution to RF estimates. We find that locally in
the source regions, a 20 % emission reduction strengthens the radiative
forcing associated with SO4 by 25 % when including the vertical
dimension, as the AFE for SO4 is strongest near the surface.
Conversely, the local RF from BC weakens by 37 % since BC AFE is low
close to the ground. The fraction of BC direct effect forcing attributable
to intercontinental transport, on the other hand, is enhanced by one-third
when accounting for the vertical aspect, because long-range transport
primarily leads to aerosol changes at high altitudes, where the BC AFE is
strong. While the surface temperature response may vary with the altitude of
aerosol change, the analysis in the present study is not extended to
estimates of temperature or precipitation changes
Numerical simulation of wellbore and formation temperature fields in carbonate formations during drilling and shut-in in the presence of lost circulation
Aerosol absorption in global models from AeroCom phase III
International audienceAbstract. Aerosol-induced absorption of shortwave radiation can modify the climate through local atmospheric heating, which affects lapse rates, precipitation, and cloud formation. Presently, the total amount of aerosol absorption is poorly constrained, and the main absorbing aerosol species (black carbon (BC), organic aerosols (OA), and mineral dust) are diversely quantified in global climate models. As part of the third phase of the Aerosol Comparisons between Observations and Models (AeroCom) intercomparison initiative (AeroCom phase III), we here document the distribution and magnitude of aerosol absorption in current global aerosol models and quantify the sources of intermodel spread, highlighting the difficulties of attributing absorption to different species. In total, 15 models have provided total present-day absorption at 550 nm (using year 2010 emissions), 11 of which have provided absorption per absorbing species. The multi-model global annual mean total absorption aerosol optical depth (AAOD) is 0.0054 (0.0020 to 0.0098; 550 nm), with the range given as the minimum and maximum model values. This is 28 % higher compared to the 0.0042 (0.0021 to 0.0076) multi-model mean in AeroCom phase II (using year 2000 emissions), but the difference is within 1 standard deviation, which, in this study, is 0.0023 (0.0019 in Phase II). Of the summed component AAOD, 60 % (range 36 %–84 %) is estimated to be due to BC, 31 % (12 %–49 %) is due to dust, and 11 % (0 %–24 %) is due to OA; however, the components are not independent in terms of their absorbing efficiency. In models with internal mixtures of absorbing aerosols, a major challenge is the lack of a common and simple method to attribute absorption to the different absorbing species. Therefore, when possible, the models with internally mixed aerosols in the present study have performed simulations using the same method for estimating absorption due to BC, OA, and dust, namely by removing it and comparing runs with and without the absorbing species. We discuss the challenges of attributing absorption to different species; we compare burden, refractive indices, and density; and we contrast models with internal mixing to models with external mixing. The model mean BC mass absorption coefficient (MAC) value is 10.1 (3.1 to 17.7) m2 g−1 (550 nm), and the model mean BC AAOD is 0.0030 (0.0007 to 0.0077). The difference in lifetime (and burden) in the models explains as much of the BC AAOD spread as the difference in BC MAC values. The difference in the spectral dependency between the models is striking. Several models have an absorption Ångstrøm exponent (AAE) close to 1, which likely is too low given current knowledge of spectral aerosol optical properties. Most models do not account for brown carbon and underestimate the spectral dependency for OA