Investigating the Influence of Anthropogenic Forcing on Observed Mean and Extreme Sea Level Pressure Trends over the Mediterranean Region

We investigate whether the observed mean sea level pressure (SLP) trends over the Mediterranean region in the period from 1975 to 2004 are significantly consistent with what 17 models projected as response of SLP to anthropogenic forcing (greenhouse gases and sulphate aerosols, GS). Obtained results indicate that the observed trends in mean SLP cannot be explained by natural (internal) variability. Externally forced changes are detectable in all seasons, except spring. The large-scale component (spatial mean) of the GS signal is detectable in all the 17 models in winter and in 12 of the 17 models in summer. However, the small-scale component (spatial anomalies about the spatial mean) of GS signal is only detectable in winter within 11 of the 17 models. We also show that GS signal has a detectable influence on observed decreasing (increasing) tendency in the frequencies of extremely low (high) SLP days in winter and that these changes cannot be explained by internal climate variability. While the detection of GS forcing is robust in winter and summer, there are striking inconsistencies in autumn, where analysis points to the presence of an external forcing, which is not GS forcing

A Recent Systematic Increase in Vapor Pressure Deficit over Tropical South America

We show a recent increasing trend in Vapor Pressure Deficit (VPD) over tropical South America in dry months with values well beyond the range of trends due to natural variability of the climate system defined in both the undisturbed Preindustrial climate and the climate over 850–1850 perturbed with natural external forcing. This trend is systematic in the southeast Amazon but driven by episodic droughts (2005, 2010, 2015) in the northwest, with the highest recoded VPD since 1979 for the 2015 drought. The univariant detection analysis shows that the observed increase in VPD cannot be explained by greenhouse-gas-induced (GHG) radiative warming alone. The bivariate attribution analysis demonstrates that forcing by elevated GHG levels and biomass burning aerosols are attributed as key causes for the observed VPD increase. We further show that There is a negative trend in evaporative fraction in the southeast Amazon, where lack of atmospheric moisture, reduced precipitation together with higher incoming solar radiation (~7% decade−1 cloud-cover reduction) influences the partitioning of surface energy fluxes towards less evapotranspiration. The VPD increase combined with the decrease in evaporative fraction are the first indications of positive climate feedback mechanisms, which we show that will continue and intensify in the course of unfolding anthropogenic climate change

Arctic marine heatwaves forced by greenhouse gases and triggered by abrupt sea-ice melt

Abstract Since 2007, unprecedented marine heatwave events are occurring over the Arctic Ocean. Here we identify the fraction of the likelihood of Arctic marine heatwaves magnitude that is attributable to greenhouse gas forcing. Results reveal that Arctic marine heatwaves are primarily triggered by an abrupt sea-ice retreat, which coincides with the maximum downward radiative fluxes. Up to 82% of the sea surface temperature variability over the shallow Arctic marginal seas, where marine heatwaves are prone to occur, can be explained by net accumulation of seasonal surface heat flux in the ocean. Event attribution analysis demonstrates that the 103-day long 2020 event – the most intense (4 ∘C) recorded so far in the Arctic – would be exceptionally unlikely in the absence of greenhouse gas forcing in terms of both intensity and duration. Our further results imply that if greenhouse gas emissions continue to rise, along with the expansion of first-year ice extent, moderate marine heatwaves in the Arctic will very likely persistently reoccur

Can Significant Trends be Detected in Surface Air Temperature and Precipitation Over South America in Recent Decades?

Trends in near-surface air temperature and precipitation over South America are examined for the periods 1975–2004 and 1955–2004, respectively, using multiple observational and climate model data sets. The results for observed near-surface air temperature show an overall warming trend over much of the continent, with the largest magnitudes over central Brazil. These observed trends are found to be statistically significant using pre-industrial control simulations from the fifth phase of the Coupled Model Intercomparison Project (CMIP5) as the baseline to estimate natural climate variability. The observed trends are compared with those obtained in natural-only CMIP5 simulations, in which only natural forcings (i.e. volcanoes and solar variability) are included, and in historical CMIP5 simulations, in which anthropogenic forcings (i.e. changes in the atmospheric composition) are further incorporated. The historical CMIP5 simulations are more successful in capturing the observed temperature trends than the simulations with natural forcings only. It is suggested that anthropogenic warming is already evident over much of South America. Unlike the warming trends, observed precipitation trends over South America are less spatially coherent with both negative and positive values across the continent. Significant positive trends are found over South America in only one of the data sets used, and over a region that roughly encompasses the southern part of La Plata Basin (southern Brazil, Uruguay, and northeastern Argentina) in all data sets used. The historical CMIP5 simulations do not capture this feature. No firm conclusions are reached, therefore, for anthropogenic influences on precipitation changes in the period selected for study

Simultaneous Regional Detection of Land-Use Changes and Elevated GHG Levels: The Case of Spring Precipitation in Tropical South America

A decline in dry season precipitation over tropical South America has a large impact on ecosystem health of the region. Results here indicate that the magnitude of negative trends in dry season precipitation in the past decades exceeds the estimated range of trends due to natural variability of the climate system defined in both the preindustrial climate and during the 850–1850 millennium. The observed drying is associated with an increase in vapor pressure deficit. The univariate detection analysis shows that greenhouse gas (GHG) forcing has a systematic influence in negative 30-year trends of precipitation ending in 1998 and later on. The bivariate attribution analysis demonstrates that forcing by elevated GHG levels and land-use change are attributed as key causes for the observed drying during 1983–2012 over the southern Amazonia and central Brazil. We further show that the effect of GS signal (GHG and sulfate aerosols) based on RCP4.5 scenario already has a detectable influence in the observed drying. Thus, we suggest that the recently observed “drier dry season” is a feature which will continue and intensify in the course of unfolding anthropogenic climate change. Such change could have profound societal and ecosystem impacts over the region

Emergent constraints on tropical atmospheric aridity - Carbon feedbacks and the future of carbon sequestration

Carbon-climate feedbacks, which amplifies or attenuates atmospheric CO2 from fossil fuel emissions, are one of the largest sources of uncertainty in climate projections. However, these feedbacks depend both on temperature and its coupling to water and energy cycles, especially in the tropics. We show that atmospheric aridity - quantified as vapor pressure deficit (VPD) - is a good proxy for this coupling. Tropical VPD is strongly correlated to the global CO2 growth rate (CGR) with observed present-day sensitivities of -2.5 ± 0.4 GtC mb-1 yr-1. The sensitivity of CGR to tropical VPD interannual variability has increased by a factor of 1.7 ± 0.3 in the 21st century. A combination of causality and statistical analysis point to mechanistic moisture drivers of the VPD-CGR sensitivities, independent of temperature. Observational records provide evidence that tropical atmospheric aridity is linked to both water deficit and spatially correlated with evaporative fraction suggesting that CGR variability is indirectly driven by land - atmosphere coupling (compound soil and atmospheric drought). This coupling is manifest as a kind of carbon-climate feedback in CMIP6 Earth System Models where long-term increases in tropical VPD reduce tropical carbon storage but with a substantial inter-model range [-1.4 to -59.4 GtC mb-1]. However, by employing a hierarchical emergent constraint, the best estimate of atmospheric aridity - carbon cycle feedback (φ TL ) is -19 ± 10 GtC mb-1, which is 28% lower than model estimates with an uncertainty reduction of 50%. Our results bridge the role of moisture and land-atmosphere coupling on net carbon variability to the vulnerability of carbon storage in a changing climate

Observed Warming Over Northern South America Has An Anthropogenic Origin

We investigate whether the recently observed trends in daily maximum and minimum near-surface air temperature (Tmax and Tmin, respectively) over South America (SA) are consistent with the simulated response of Tmin and Tmax to anthropogenic forcing. Results indicate that the recently observed warming in the dry seasons is well beyond the range of natural (internal) variability. In the wet season the natural modes of variability explain a substantial portion of Tmin and Tmax variability. We demonstrate that the large-scale component of greenhouse gas (GHG) forcing is detectable in dry-seasonal warming. However, none of the global and regional climate change projections reproduce the observed warming of up to 0.6 K/Decade in Tmax in 1983–2012 over northern SA during the austral spring (SON). Thus, besides the global manifestation of GHG forcing, other external drivers have an imprint. Using aerosols-only forcing simulations, our results provide evidence that anthropogenic aerosols also have a detectable influence in SON and that the indirect effect of aerosols on cloud’s lifetime is more compatible with the observed record. In addition, there is an increasing trend in the observed incoming solar radiation over northern SA in SON, which is larger than expected from natural (internal) variability alone. We further show that in the dry seasons the spread of projected trends based on the RCP4.5 scenario derived from 30 CMIP5 models encompasses the observed area-averaged trends in Tmin and Tmax. This may imply that the observed excessive warming in the dry seasons serve as an illustration of plausible future expected change in the region

A Recent Systematic Increase in Vapor Pressure Deficit over Tropical South America

We show a recent increasing trend in Vapor Pressure Deficit (VPD) over tropical South America in dry months with values well beyond the range of trends due to natural variability of the climate system defined in both the undisturbed Preindustrial climate and the climate over 850-1850 perturbed with natural external forcing. This trend is systematic in the southeast Amazon but driven by episodic droughts (2005, 2010, 2015) in the northwest, with the highest recoded VPD since 1979 for the 2015 drought. The univariant detection analysis shows that the observed increase in VPD cannot be explained by greenhouse-gas-induced (GHG) radiative warming alone. The bivariate attribution analysis demonstrates that forcing by elevated GHG levels and biomass burning aerosols are attributed as key causes for the observed VPD increase. We further show that There is a negative trend in evaporative fraction in the southeast Amazon, where lack of atmospheric moisture, reduced precipitation together with higher incoming solar radiation (~7% decade-1 cloud-cover reduction) influences the partitioning of surface energy fluxes towards less evapotranspiration. The VPD increase combined with the decrease in evaporative fraction are the first indications of positive climate feedback mechanisms, which we show that will continue and intensify in the course of unfolding anthropogenic climate change.U.S. National Science FoundationNational Science Foundation (NSF) [AGS-1547899]; Jet Propulsion Laboratory, California Institute of Technology, under a terrestrial ecology and carbon cycle program grant [WBS: 596741.02.01.01.67]Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]