Yangyang Xu

Solar absorption by elemental and brown carbon determined from spectral observations

Black carbon (BC) is functionally defined as the absorbing component of atmospheric total carbonaceous aerosols (TC) and is typically dominated by soot-like elemental carbon (EC). However, organic carbon (OC) has also been shown to absorb strongly at visible to UV wavelengths and the absorbing organics are referred to as brown carbon (BrC), which is typically not represented in climate models. We propose an observationally based analytical method for rigorously partitioning measured absorption aerosol optical depths (AAOD) and single scattering albedo (SSA) among EC and BrC, using multiwavelength measurements of total (EC, OC, and dust) absorption. EC is found to be strongly absorbing (SSA of 0.38) whereas the BrC SSA varies globally between 0.77 and 0.85. The method is applied to the California region. We find TC (EC + BrC) contributes 81% of the total absorption at 675 nm and 84% at 440 nm. The BrC absorption at 440 nm is about 40% of the EC, whereas at 675 nm it is less than 10% of EC. We find an enhanced absorption due to OC in the summer months and in southern California (related to forest fires and secondary OC). The fractions and trends are broadly consistent with aerosol chemical-transport models as well as with regional emission inventories, implying that we have obtained a representative estimate for BrC absorption. The results demonstrate that current climate models that treat OC as nonabsorbing are underestimating the total warming effect of carbonaceous aerosols by neglecting part of the atmospheric heating, particularly over biomass-burning regions that emit BrC.

The Copenhagen Accord for limiting global warming: Criteria, constraints, and available avenues

At last, all the major emitters of greenhouse gases (GHGs) have agreed under the Copenhagen Accord that global average temperature increase should be kept below 2 °C. This study develops the criteria for limiting the warming below 2 °C, identifies the constraints imposed on policy makers, and explores available mitigation avenues. One important criterion is that the radiant energy added by human activities should not exceed 2.5 (range: 1.7–4) watts per square meter (Wm−2) of the Earths surface. The blanket of man-made GHGs has already added 3 (range: 2.6–3.5) Wm−2. Even if GHG emissions peak in 2015, the radiant energy barrier will be exceeded by 100%, requiring simultaneous pursuit of three avenues: (i) reduce the rate of thickening of the blanket by stabilizing CO2 concentration below 441 ppm during this century (a massive decarbonization of the energy sector is necessary to accomplish this Herculean task), (ii) ensure that air pollution laws that reduce the masking effect of cooling aerosols be made radiant energy-neutral by reductions in black carbon and ozone, and (iii) thin the blanket by reducing emissions of short-lived GHGs. Methane and hydrofluorocarbons emerge as the prime targets. These actions, even if we are restricted to available technologies for avenues ii and iii, can reduce the probability of exceeding the 2 °C barrier before 2050 to less than 10%, and before 2100 to less than 50%. With such actions, the four decades we have until 2050 should be exploited to develop and scale-up revolutionary technologies to restrict the warming to less than 1.5 °C.

Does extreme precipitation intensity depend on the emissions scenario

The rate of increase of global-mean precipitation per degree global-mean surface temperature increase differs for greenhouse gas and aerosol forcings and across emissions scenarios with differing composition of change in forcing. We investigate whether or not the rate of change of extreme precipitation also varies across the four emissions scenarios that force the CMIP5 multi-model ensemble. In most models, the rate of increase of maximum annual daily precipitation per degree global warming in the multi-model ensemble is statistically indistinguishable across the four scenarios, whether this extreme precipitation is calculated globally, over all land, or over extra-tropical land. These results indicate that, in contrast to mean precipitation, extreme precipitation depends on the total amount of warming and does not depend on emissions scenario in most models.

Simulated differences in 21st century aridity due to different scenarios of greenhouse gases and aerosols

Aridity, defined as the ratio of precipitation (P) to potential evapotranspiration (PET) over land, is critical to natural ecosystems and agricultural production. Global climate models project global decreases of P/PET (drying) in the 21st century. We examine the uncertainty of aridity projections due to scenarios of greenhouse gases (GHGs) and aerosols with three sets of ensemble simulations from a single climate model, the Community Earth System Model (CESM1). Ensembles consist of two Radiative Concentration Pathways (RCPs) and a scenario with RCP-like GHGs but with aerosol precursor emissions and atmospheric oxidants fixed at the year 2005 level. Under a high GHGs emission scenario (RCP8.5), global land P/PET decreases (drying) by 6.4xa0±xa00.8xa0% in 2060–2080 relative to 1985–2005. A GHG mitigation scenario (RCP4.5) would reduce the drying (P/PET decrease) to 3.7xa0±xa00.6xa0%. Although future aerosol emissions reduction would increase P, we find that it has little impact on global aridity due to offsetting effects on PET. Regionally, deceasing aerosols can have significant effects and aerosol-induced P/PET changes are due to different factors across different regions. When normalized by global mean temperature response, GHGs decrease global land P/PET by 2.7xa0±xa00.6xa0%/°C and surface temperature changes dominate GHG-induced P/PET change.

The importance of aerosol scenarios in projections of future heat extremes

Global climate models project a large increase in the frequency and intensity of heat extremes (HEs) during the 21st century under the Representative Pathway Concentration (RCP8.5) scenario. To assess the relative sensitivity of future HEs to the level of greenhouse gas (GHG) increases and aerosol emission decreases, we contrast Community Earth System Model (CESM)’s Large Ensemble projection under RCP8.5 with two additional ensembles: one keeping aerosol emissions at 2005 levels (but allowing all other forcings to progress as in RCP8.5) and the other using the RCP4.5 with lower GHG levels. By the late 21st century (2060–2080), the 3xa0°C warmer-than-present-day climate simulated under RCP8.5 could be 0.6xa0°C cooler (0.9xa0°C over land) if the aerosol emissions in RCP8.5 were not reduced, compared with a 1.2xa0°C cooling due to GHG mitigation (switching from RCP8.5 to RCP4.5). Aerosol induced cooling and associated HE reductions are relatively stronger in the Northern Hemisphere (NH), as opposed to GHG mitigation induced cooling. When normalized by the global mean temperature change in these two cases, aerosols have a greater effect than GHGs on all HE statistics over NH extra-tropical land areas. Aerosols are more capable of changing HE duration than GHGs in the tropics, explained by stronger dynamical changes in atmospheric circulation, despite weaker thermodynamic changes. Our results highlight the importance of aerosol scenario assumptions in projecting future HEs at regional scales.

Latitudinally asymmetric response of global surface temperature: Implications for regional climate change

[1]xa0The Earths climate system was subject to two multi-decadal warming trends in the beginning (1910–1940) and end (1975–2005) of the 20th century, having been interrupted only by a cooling trend in mid-century (1940–1975). The spatio-temporal distribution of surface temperature during this time, especially the land-ocean warming contrast in recent decades, has been the subject of many climate change detection studies. The focus of this study is the south-to-north warming asymmetry and we observed a similar Latitudinal Asymmetry of Temperature Change (LATC) for the two warming sub-periods and the cooling sub-period. Basically, the temperature change was low in the Southern Hemisphere extra-tropics (60°S) and increased monotonically to peak values (0.15°C/decade for warming trends) in the Northern Hemisphere extra-tropics (60°N). We hypothesized that the LATC is a fundamental characteristic of the planets transient response to global forcing. We tested this hypothesis using climate model simulations of CO2 and aerosol forcing, and the simulations revealed very similar LATC as seen in the observations. In the simulations, the LATC did not depend on the asymmetry of the forcing and furthermore weakened significantly in equilibrium simulations, leading to the deduction that the LATC was caused by a corresponding asymmetry in the land-ocean fraction, i.e., the analyses of model simulations supported the hypothesis of LATC being a fundamental characteristic of the planets transient response. If LATC is preserved as the planet warms beyond 2°C, precipitation patterns can be drastically disrupted in the tropics and sub-tropics, with major implications for regional climate.

Well below 2 °C: Mitigation strategies for avoiding dangerous to catastrophic climate changes

The historic Paris Agreement calls for limiting global temperature rise to “well below 2 °C.” Because of uncertainties in emission scenarios, climate, and carbon cycle feedback, we interpret the Paris Agreement in terms of three climate risk categories and bring in considerations of low-probability (5%) high-impact (LPHI) warming in addition to the central (∼50% probability) value. The current risk category of dangerous warming is extended to more categories, which are defined by us here as follows: >1.5 °C as dangerous; >3 °C as catastrophic; and >5 °C as unknown, implying beyond catastrophic, including existential threats. With unchecked emissions, the central warming can reach the dangerous level within three decades, with the LPHI warming becoming catastrophic by 2050. We outline a three-lever strategy to limit the central warming below the dangerous level and the LPHI below the catastrophic level, both in the near term (<2050) and in the long term (2100): the carbon neutral (CN) lever to achieve zero net emissions of CO2, the super pollutant (SP) lever to mitigate short-lived climate pollutants, and the carbon extraction and sequestration (CES) lever to thin the atmospheric CO2 blanket. Pulling on both CN and SP levers and bending the emissions curve by 2020 can keep the central warming below dangerous levels. To limit the LPHI warming below dangerous levels, the CES lever must be pulled as well to extract as much as 1 trillion tons of CO2 before 2100 to both limit the preindustrial to 2100 cumulative net CO2 emissions to 2.2 trillion tons and bend the warming curve to a cooling trend.

The Benefits of Reduced Anthropogenic Climate changE (BRACE): a synthesis

Understanding how impacts may differ across alternative levels of future climate change is necessary to inform mitigation and adaptation measures. The Benefits of Reduced Anthropogenic Climate changE (BRACE) project assesses the differences in impacts between two specific climate futures: a higher emissions future with global average temperature increasing about 3.7xa0°C above pre-industrial levels toward the end of the century and a moderate emissions future with global average warming of about 2.5xa0°C. BRACE studies in this special issue quantify avoided impacts on physical, managed, and societal systems in terms of extreme events, health, agriculture, and tropical cyclones. Here we describe the conceptual framework and design of BRACE and synthesize its results. Methodologically, the project combines climate modeling, statistical analysis, and impact assessment and draws heavily on large ensembles using the Community Earth System Model. It addresses uncertainty in future societal change by employing two pathways for future socioeconomic development. Results show that the benefits of reduced climate change within this framework vary substantially across types of impacts. In many cases, especially related to extreme heat events, there are substantial benefits to mitigation. The benefits for some heat extremes are statistically significant in some regions as early as the 2020s and are widespread by mid-century. Benefits are more modest for agriculture and exposure to some health risks. Benefits are negative for agriculture when CO2 fertilization is incorporated. For several societal impacts, the effect on outcomes of alternative future societal development pathways is substantially larger than the effect of the two climate scenarios.

How Would the Twenty-First-Century Warming Influence Pacific Decadal Variability and Its Connection to North American Rainfall: Assessment Based on a Revised Procedure for the IPO/PDO

AbstractDecadal climate variability of sea surface temperature (SST) over the Pacific Ocean can be characterized by Interdecadal Pacific Oscillation (IPO) or Pacific Decadal Oscillation (PDO) based on Empirical Orthogonal Function (EOF) analysis. Although the procedures to derive the IPO and PDO indices differ in their regional focuses and filtering methods to remove interannual variability, IPO and PDO are highly correlated in time and are often used interchangeably. Studies have shown that the IPO/PDO play a vital role in modulating the pace of global warming. It is less clear, however, how externally-forced global warming may, in turn, affect IPO/PDO. One obstacle to revealing this effect is that the conventional definitions of IPO/PDO fail to account for the spatial heterogeneity of background warming trend, which causes IPO/PDO to be conflated with the warming trend, especially for the 21st-century simulation when the forced change is likely to be more dominant. Using a large-ensemble simulation in C...

Pattern scaling based projections for precipitation and potential evapotranspiration: sensitivity to composition of GHGs and aerosols forcing

Pattern scaling is a computationally efficient method to generate global projections of future climate changes, such as temperature and precipitation, under various emission scenarios. In this study, we apply the pattern-scaling method to project future changes of potential evapotranspiration (PET), a metric highly relevant to hydroclimate research. While doing so, this study tests the basic assumption of pattern-scaling methods, which is that the underlying scaling pattern is largely identical across all emission scenarios. We use a pair of the large-ensemble global climate model (GCM) simulations and obtain the two separate scaling patterns, one due to greenhouse gasses (GHGs) and the other due to aerosols, which show substantial regional differences. We also derive a single combined pattern, encapsulating the effects of both forcings. Using an energy balance climate model, future changes in temperature, precipitation, and PET are projected by combining the separate GHGs and aerosols scaling patterns (“hybrid-pattern” approach) and the performance of this “hybrid-pattern” approach is compared to the conventional approach (“single-pattern”) by evaluating both approaches against the GCM direct output. We find that both approaches provide reasonably good emulations for the long-term projection (end of the twenty-first century). However, the “hybrid-pattern” approach provides better emulations for the near-term climate changes (2020–2040) when the large changes in aerosol emissions occur.