Intensification of seasonal extremes given a 2°C global warming target

Current international efforts to reduce greenhouse gas emissions and limit human-induced global-mean near-surface temperature increases to 2°C, relative to the pre-industrial era, are intended to avoid possibly significant and dangerous impacts to physical, biological, and socio-economic systems. However, it is unknown how these various systems will respond to such a temperature increase because their relevant spatial scales are much different than those represented by numerical global climate models—the standard tool for climate change studies. This deficiency can be addressed by using higher-resolution regional climate models, but at great computational expense. The research presented here seeks to determine how a 2°C global-mean temperature increase might change the frequency of seasonal temperature extremes, both in the United States and around the globe, without necessarily resorting to these computationally-intensive model experiments. Results indicate that in many locations the regional temperature increases that accompany a 2°C increase in global mean temperatures are significantly larger than the interannual-to-decadal variations in seasonal-mean temperatures; in these locations a 2°C global mean temperature increase results in seasonal-mean temperatures that consistently exceed the most extreme values experienced during the second half of the 20th Century. Further, results indicate that many tropical regions, despite having relatively modest overall temperature increases, will have the most substantial increase in number of hot extremes. These results highlight that extremes very well could become the norm, even given the 2°C temperature increase target.     

Assessing the strength of regional changes in near-surface climate associated with a global warming of 2°C

In this study, the strength of the regional changes in near-surface climate associated with a global warming of 2°C with respect to pre-industrial times is assessed, distinguishing between 26 different regions. Also, the strength of these regional climate changes is compared to the strength of the respective changes associated with a markedly stronger global warming of 4.5°C. The magnitude of the regional changes in climate is estimated by means of a normalized regional climate change index, which considers changes in the mean as well as changes in the interannual variability of both near-surface temperature and precipitation. The study is based on two sets of four ensemble simulations with the ECHAM5/MPI-OM coupled climate model, each starting from different initial conditions. In one set of simulations (1860–2200), the greenhouse gas concentrations and sulphate aerosol load have been prescribed according to observations until 2000 and according to the SRES A1B scenario after 2000. In the other set of simulations (2020–2200), the greenhouse gas concentrations and sulphate aerosol load have been prescribed in such a way that the simulated global warming does not exceed 2°C with respect to pre-industrial times. The study reveals the strongest changes in near-surface climate in the same regions for both scenarios, i.e., the Sahara, Northern Australia, Southern Australia and Amazonia. The regions with the weakest changes in near-surface climate, on the other hand, vary somewhat between the two scenarios except for Western North America and Southern South America, where both scenarios show rather weak changes. The comparison between the magnitude of the regional changes in near-surface climate for the two scenarios reveals relatively strong changes in the 2°C-stabilization scenario at high northern latitudes, i.e., Northeastern Europe, Alaska and Greenland, and in Amazonia. Relatively weak regional climate changes in this scenario, on the other hand, are found for Eastern Asia, Central America, Central South America and Southern South America. The ratios between the regional changes in the near-surface climate for the two scenarios vary considerably between different regions. This illustrates a limitation of obtaining regional changes in near-surface climate associated with a particular scenario by means of scaling the regional changes obtained from a widely used “standard” scenario with the ratio of the changes in the global mean temperature projected by these two scenarios.     

Projected changes in summer water vapor transport over East Asia under the 1.5°C and 2.0°C global warming targets

水汽输送的变化对于降水的变化有重要贡献。基于优选的13个CIV1IP5模式发现:RCP4.5和RCP8.5排放情景下,1.5°C和2.0°C增暖时东亚夏季水汽输送均加强,且2.0°C增暖时模式间一致性更好;水汽含量的增加对东亚南部和北部水汽输送的加强均有贡献,东亚南部水汽输送的加强也与低层环流的加强相联系。0.5°C额外增暖(1.5°C和2.0°C增暖间比较)时,两种排放情景下水汽输送的变化在我国南海与东北地区存在差异,使得两个地区降水变化存在差异;水汽输送的变化与低层环流的变化关系密切,且模式间一致性相对低。     

Mapping the vulnerability of European summer tourism under 2 °C global warming

Summer tourism is one of the most important contributors to the European GDP especially for the southern countries and is highly dependent on the climatic conditions. Changes in average climatic conditions, along with the potential subsequent changes in the physical environment, will pose stress on the favorability of the climate of European destinations for tourism and recreational activities. Here, we study the vulnerability of summer-oriented tourism due to a global temperature increase by 2 °C relative to the preindustrial era. We use a well-defined framework of exposure, sensitivity, and adaptive capacity indicators for a set of plausible climate (RCPs) and socioeconomic (SSPs) combinations. Our result shows that a 2 °C global warming will pose substantial changes to the vulnerability of the European tourism sector. Despite the general increase in exposure, the vulnerability of summer tourism is highly depended on the socioeconomic developments (SSPs). Although exposure is higher for most of the popular southern European destinations like Spain, France, South Italy, southernmost Greece, and Cyprus, they are expected to be less vulnerable than others, under specific SSPs, due to their higher capacity to adapt to a different climate. The capacity to adapt is lower for higher emission scenarios. Substantial changes are also apparent at the subnational level. Countries like France are foreseen to experience very diverse impacts and vulnerabilities within their own territories that will have consequences in terms of domestic tourism. The dynamics of these changes are expected to alter the state of the current European tourism regime.     

Projections of 2.0°C Warming over the Globe and China under RCP4.5

The outputs of 17 models in the Coupled Model Intercomparison Project Phase 5 (CMIP5) are employed to investigate the temporal and spatial features of 2.0°C warming of the surface temperature over the globe and China under the Representative Concentration Pathways (RCP) 4.5 scenario. The simulations of the period 1860–1899 in the historical experiment are chosen as the baseline. The simulations for the 21st century in the RCP4.5 experiment are chosen as the future project. The multi-model ensemble mean (MME) shows that the global mean temperature would cross the 2.0°C warming threshold in 2047. Warming in most of the models would cross the threshold during 2030–2060. For local warming, high-latitude areas in the Northern Hemisphere show the fastest warming over the globe. Land areas warm substantially faster than the oceans. Most of the southern oceans would not exceed the 2.0°C warming threshold within the 21st century. Over China, surface warming is substantially faster than the global mean. The area-averaged warming would cross the 2.0°C threshold in 2034. Locally, Northwest China shows the fastest warming trend, followed by Central North China and Northeast China. Central China, East China, and South China are the last to cross the 2.0°C warming threshold. The diversity of the models is also estimated in this study. Generally, the spread among the models increases with time, and there is smaller spread among the models for the areas with the faster warming.     

Near-term increase in frequency of seasonal temperature extremes prior to the 2°C global warming target

Given current international efforts to reduce greenhouse gas emissions and limit human-induced global-mean near-surface temperature increases to 2°C, relative to the pre-industrial era, we seek to determine the impact such a temperature increase might have upon the frequency of seasonal-mean temperature extremes; further we seek to determine what global-mean temperature increase would prevent extreme temperature values from becoming the norm. Results indicate that given a 2°C global mean temperature increase it is expected that for 70–80% of the land surface maximum seasonal-mean temperatures will exceed historical extremes (as determined from the 95th percentile threshold value over the second half of the 20th Century) in at least half of all years, i.e. the current historical extreme values will effectively become the norm. Many regions of the globe—including much of Africa, the southeastern and central portions of Asia, Indonesia, and the Amazon—will reach this point given the “committed” future global-mean temperature increase of 0.6°C (1.4°C relative to the pre-industrial era) and 50% of the land surface will reach it given a future global-mean temperature increase of between 0.8 and 0.95°C (1.6–1.75°C relative to the pre-industrial era). These results suggest substantial fractions of the globe could experience seasonal-mean temperature extremes with high regularity, even if the global-mean temperature increase remains below the 2°C target.     

Sharing the reduction effort to limit global warming to 2°C

In order to stabilize long-term greenhouse gas concentrations at 450 ppm CO₂-eq or lower, developed countries as a group should reduce emissions by 25–40% below 1990 levels by 2020, while developing countries' emissions need to be reduced by around 15–30%, relative to their baseline levels, according to the IPCC and our earlier work. This study examines 19 other studies on the emission reductions attributed to the developed and developing countries for meeting a 450 ppm target. These studies considered different allocation approaches, according to equity principles. The effect of the assumed global emissions cap in these studies is analysed. For developed countries, the original reduction range of 25–40% by 2020 is still within the average range of all studies, but does not cover it completely. Comparing the studies shows that assuming a global emissions cap of 5–15% above 1990 levels by 2020 generally leads to more stringent reduction targets than when a global emissions cap of 20–30% above 1990 levels is assumed. For developing countries, the reduction range of 15–30% below their baseline levels by 2020 corresponds to an increase on the 1990 level from 70% (about the 2006 level) to 120%. Reducing deforestation emissions by 50% below baseline levels by 2020 may relax the emission reductions for either group of countries; for developing countries by about 7% or for developed countries by about 15% (but not for both).     

China's role in attaining the global 2°C target

In the recent climate change negotiations it was declared that the increase in global temperature should be kept below 2°C by 2100, relative to pre-industrial levels. China's CO₂ emissions from energy and cement processes already account for nearly 24% of global emissions, a trend that is expected to keep increasing. Thus the role of China in global GHG mitigation is crucial. A scenario analysis of China's CO₂ emissions is presented here and the feasibility of China reaching a low-carbon scenario is discussed. The results suggest that recent and continued technological progress will make it possible for China to limit its CO₂ emissions and for these emissions to peak before 2025 and therefore that the global 2°C target can be achieved.

Policy relevance

In signing the Copenhagen Accord, China agreed to the global 2°C target. Results from this article could be used to justify low-carbon development policies and negotiations. While many still doubt the feasibility of a low-carbon pathway to support the global 2°C target, the results suggest that such a pathway can be realistically achieved. This conclusion should increase confidence and guide the policy framework further to make possible China's low-carbon development. Related policies and measures, such as renewable energy development, energy efficiency, economic structure optimization, technology innovation, low-carbon investment, and carbon capture and storage (CCS) development, should be further enhanced. Furthermore, China can play a larger role in the international negotiations process. In the global context, the 2°C target could be reaffirmed and a global regime on an emissions mitigation protocol could be framed with countries’ emissions target up to 2050.     

Uncertainty in the 2°C warming threshold related to climate sensitivity and climate feedback

Climate sensitivity is an important index that measures the relationship between the increase in greenhouse gases and the magnitude of global warming. Uncertainties in climate change projection and climate modeling are mostly related to the climate sensitivity. The climate sensitivities of coupled climate models determine the magnitudes of the projected global warming. In this paper, the authors thoroughly review the literature on climate sensitivity, and discuss issues related to climate feedback processes and the methods used in estimating the equilibrium climate sensitivity and transient climate response (TCR), including the TCR to cumulative CO₂ emissions. After presenting a summary of the sources that affect the uncertainty of climate sensitivity, the impact of climate sensitivity on climate change projection is discussed by addressing the uncertainties in 2°C warming. Challenges that call for further investigation in the research community, in particular the Chinese community, are discussed.     

Precipitation response to La Niña and global warming in the Indo-Pacific

Recent studies have highlighted the nonlinear rainfall response to El Niño sea surface temperature (SST) events in the Indo-Pacific region and how this response might change over coming decades. Here we investigate the response to La Niña SST anomalies with and without global warming by performing idealised SST-forced experiments with an atmospheric general circulation model. The La Niña SST anomaly is multiplied by a factor \(1 \le \alpha \le 4\) and added to climatological SSTs. Similar experiments using El Niño SST anomalies were previously performed, in which large nonlinearities in the precipitation response were evident. We find that: (i) Under current climatic conditions, as \(\alpha\) increases, the precipitation responds in three ways: the intertropical convergence zone (ITCZ) dries and moves poleward, the maximum precipitation along the equator moves west, and the South Pacific convergence zone (SPCZ) narrows, intensifies, and elongates. For weak ( \(\alpha = 1\) ) La Niña events, the precipitation anomalies approximately mirror those from the El Niño events along the ITCZ and SPCZ, though there are some marked differences in the central-eastern Pacific. For stronger La Niña events ( \(\alpha > 1\) ), precipitation responds nonlinearly to SST anomalies, though the nonlinearities are smaller and differ spatially from the nonlinearities in the El Niño runs. (ii) The addition of a global warming SST pattern increases rainfall in the western Pacific and SPCZ, enhances the narrowing of the SPCZ, and increases the nonlinear response in the western Pacific. However, large La Niña events reduce the impact of global warming along the central-eastern equatorial Pacific as the global warming and La Niña SST anomalies have opposite signs in that region. (iii) The response to La Niña SST anomalies is driven primarily by changes in the atmospheric circulation, whereas the response to the global warming SST pattern is mainly driven by increases in atmospheric moisture. (iv) Large changes in La Niña-driven rainfall anomalies can occur in response to global warming, even if the La Nina SST anomalies relative to the warmer background state are completely unchanged.     

Is there a common pattern of future gas-phase air pollution in Europe under diverse climate change scenarios?

Climate change alone influences future levels of tropospheric ozone and their precursors through modifications of gas-phase chemistry, transport, removal, and natural emissions. The goal of this study is to determine at what extent the modes of variability of gas-phase pollutants respond to different climate change scenarios over Europe. The methodology includes the use of the regional modeling system MM5 (regional climate model version)-CHIMERE for a target domain covering Europe. Two full-transient simulations covering from 1991–2050 under the SRES A2 and B2 scenarios driven by ECHO-G global circulation model have been compared. The results indicate that the spatial patterns of variability for tropospheric ozone are similar for both scenarios, but the magnitude of the change signal significantly differs for A2 and B2. The 1991–2050 simulations share common characteristics for their chemical behavior. As observed from the NO₂ and α-pinene modes of variability, our simulations suggest that the enhanced ozone chemical activity is driven by a number of parameters, such as the warming-induced increase in biogenic emissions and, to a lesser extent, by the variation in nitrogen dioxide levels. For gas-phase pollutants, the general increasing trend for ozone found under A2 and B2 forcing is due to a multiplicity of climate factors, such as increased temperature, decreased wet removal associated with an overall decrease of precipitation in southern Europe, increased photolysis of primary and secondary pollutants as a consequence of lower cloudiness and increased biogenic emissions fueled by higher temperatures.     

Does global warming favour the occurrence of extreme floods in European Alps? First evidences from a NW Alps proglacial lake sediment record

Flood hazard is expected to increase in the context of global warming. However, long time-series of climate and gauge data at high-elevation are too sparse to assess reliably the rate of recurrence of such events in mountain areas. Here paleolimnological techniques were used to assess the evolution of frequency and magnitude of flash flood events in the North-western European Alps since the Little Ice Age (LIA). The aim was to document a possible effect of the post-19^th century global warming on torrential floods frequency and magnitude. Altogether 56 flood deposits were detected from grain size and geochemical measurements performed on gravity cores taken in the proglacial Lake Blanc (2170?m?a.s.l., Belledonne Massif, NW French Alps). The age model relies on radiometric dating (¹³⁷Cs and ²⁴¹Am), historic lead contamination and the correlation of major flood- and earthquake-triggered deposits, with recognized occurrences in historical written archives. The resulting flood calendar spans the last ca 270?years (AD 1740–AD 2007). The magnitude of flood events was inferred from the accumulated sediment mass per flood event and compared with reconstructed or homogenized datasets of precipitation, temperature and glacier variations. Whereas the decennial flood frequency seems to be independent of seasonal precipitation, a relationship with summer temperature fluctuations can be observed at decadal timescales. Most of the extreme flood events took place since the beginning of the 20^th century with the strongest occurring in 2005. Our record thus suggests climate warming is favouring the occurrence of high magnitude torrential flood events in high-altitude catchments.     

Future frost risks in the Tohoku region of Japan under a warming climate—interpretation of regional diversity in terms of seasonal warming

Theoretical and Applied Climatology - We investigated future frost risks in the Tohoku Region of Japan under climate change. We focused on the processes governing regional variations in the future...     

The sensitivity of the Indian summer monsoon to a global warming of 2��C with respect to pre-industrial times

In this study the potential future changes in different aspects of the Indian summer monsoon associated with a global warming of 2°C with respect to pre-industrial times are assessed, focussing on the role of the different mechanisms leading to these changes. In addition, these changes as well as the underlying mechanisms are compared to the corresponding changes associated with a markedly stronger global warming exceeding 4.5°C, associated with the widely used SRES A1B scenario. The study is based on two sets of four ensemble simulations with the ECHAM5/MPI-OM coupled climate model, each starting from different initial conditions. In one set of simulations (2020?C2200), greenhouse gas concentrations and sulphate aerosol load have been prescribed in such a way that the simulated global warming dioes not exceed 2°C with respect to pre-industrial times. In the other set of simulations (1860?C2200), greenhouse gas concentrations and sulphate aerosol load have been prescribed according to observations until 2000 and according to the SRES A1B scenario after 2000. The study reveals marked changes in the Indian summer monsoon associated with a global warming of 2°C with respect to pre-industrial conditions, namely an intensification of the summer monsoon precipitation despite a weakening of the large-scale monsoon circulation. The increase in the monsoon rainfall is related to a variety of different mechanisms, with the intensification of the atmospheric moisture transport into the Indian region as the most important one. The weakening of the large-scale monsoon circulation is mainly caused by changes in the Walker circulation with large-scale divergence (convergence) in the lower (uppper) troposphere over the Indian Ocean in response to enhanced convective activity over the Indian Ocean and the central and eastern Pacific and reduced convective activity over the western tropical Pacific. These changes in the Walker circulation induce westerly (easterly) wind anomalies at lower (upper) level in the Indian region. The comparison with the changes in the Indian summer monsoon associated with a global warming of 4.5°C reveals that both the intensification of the monsoon precipitation and the weakening of the large-scale monsoon circulation (particularly in the lower troposphere) are relatively strong (with respect to the magnitude of the projected global warming by the end of the twentieth century for the two scenarios) in the scenario with a global warming of 2°C. The relatively strong intensification of the monsoon rainfall is related to rather strong increases in evaporation over the Arabian Sea and the Bay of Bengal, while a rather weak amplification of the meridional temperature gradient between the Indian Ocean and the land areas to the north contributes to the relatively strong reduction of the large-scale monsoon flow.     

Environmental and resource footprints in a global context: Europe’s structural deficit in resource endowments

The European Union (EU) has proposed in its Resource-efficiency roadmap a ‘dashboard of indicators’ consisting of four headline indicators for carbon, water, land and materials. The EU recognizes the need to use a consumption-based (or ‘footprint’) perspective to capture the global dimension of resources and their impacts. In this paper, we analyse how the EU’s footprints compare to those of other nations, to what extent the EU and other major economies of the world rely on embodied resource imports, and what the implications are for policy making based on this comparison. This study is the first comprehensive multi-indicator comparison of all four policy relevant indicators, and uses a single consistent global Multi-Regional Input Output (MRIO) database with a unique and high level of product detail across countries. We find that Europe is the only region in the world that relies on net embodied imports for all indicators considered. We further find that the powerful economies of China and others in the Asia-Pacific already dominate global resource consumption from a footprint perspective, while they still haven’t reached the prosperity of developed countries. Competition for resources is hence likely to increase, making Europe even more vulnerable. A hot spot analysis suggests that final consumption of food, transport and housing are priorities for reduction efforts along the life cycle. Further, countries with a similar Human Development Index can have very different footprints, pointing at societal organisation at macro-level as option for improvement. This points at options for countries for lowering their footprint, becoming less dependent on embodied imports, while maintaining a high quality of life.     

Meeting the EU 2°C climate target: global and regional emission implications

Abstract

This article presents a set of multi-gas emission pathways for different CO₂-equivalent concentration stabilization levels, i.e. 400, 450, 500 and 550 ppm CO₂-equivalent, along with an analysis of their global and regional reduction implications and implied probability of achieving the EU climate target of 2°C. For achieving the 2°C target with a probability of more than 60%, greenhouse gas concentrations need to be stabilized at 450 ppm CO₂-equivalent or below, if the 90% uncertainty range for climate sensitivity is believed to be 1.5–4.5°C. A stabilization at 450 ppm CO₂-equivalent or below (400 ppm) requires global emissions to peak around 2015, followed by substantial overall reductions of as much as 25% (45% for 400 ppm) compared to 1990 levels in 2050. In 2020, Annex I emissions need to be approximately 15% (30%) below 1990 levels, and non-Annex I emissions also need to be reduced by 15–20% compared to their baseline emissions. A further delay in peaking of global emissions by 10 years doubles maximum reduction rates to about 5% per year, and very probably leads to high costs. In order to keep the option open of stabilizing at 400 and 450 ppm CO₂-equivalent, the USA and major advanced non-Annex I countries will have to participate in the reductions within the next 10–15 years.     

Comparing future patterns of energy system change in 2 °C scenarios with historically observed rates of change

This paper systematically compares modeled rates of change provided by global integrated assessment models aiming for the 2 °C objective to historically observed rates of change. Such a comparison can provide insights into the difficulty of achieving such stringent climate stabilization scenarios. The analysis focuses specifically on the rates of change for technology expansion and diffusion, emissions and energy supply investments. The associated indicators vary in terms of system focus (technology-specific or energy system wide), temporal scale (timescale or lifetime), spatial scale (regional or global) and normalization (accounting for entire system growth or not). Although none of the indicators provide conclusive insights as to the achievability of scenarios, this study finds that indicators that look into absolute change remain within the range of historical growth frontiers for the next decade, but increase to unprecedented levels before mid-century. Indicators that take into account or normalize for overall system growth find future change to be broadly within historical ranges. This is particularly the case for monetary-based normalization metrics like GDP compared to energy-based normalization metrics like primary energy. By applying a diverse set of indicators alternative, complementary insights into how scenarios compare with historical observations are acquired but they do not provide further insights on the possibility of achieving rates of change that are beyond current day practice.     

Interdecadal aridity variations in Central Asia during 1950–2016 regulated by oceanic conditions under the background of global warming

As a typical inland arid and semiarid region, Central Asia (CA) is vulnerable to the forced global warming (FGW) due to anthropogenic activity. Aiming at the interdecadal variation of the FGW-forced aridity pattern (FAP) in CA, we try to extract the associated oceanic and atmospheric modes by analyzing observations, reanalysis data and multi-model simulations during 1950–2016. The FAP in CA features a tripolar pattern with wetting–drying-wetting responses arranging from southeast to northwest and shows strong interdecadal-to-interannual amplitude variations. It is found that the sea surface temperature (SST) in the tropical South Atlantic (TSA) well correlates with the amplitude variation of FAP on interdecadal time scale, possibly through modulating the interannual SST modes characterized by the North Atlantic horseshoe-like dipole (NAHD) and the El Ninõ and South Oscillation (ENSO). Corresponding to the enhancing FAP from the middle 1970s to early 2000s, the TSA-modulated NAHD and ENSO, together with the Pacific Decadal Oscillation-modulated Indian Ocean Dipole-like mode, show connections with an Eurasian middle-latitude wave train coupled with the North Arctic Oscillation and equatorial low, which favors the moisture transport to strengthen the tripolar FAP by forming a local circulation dipole with positive/negative anomaly over the northwest/southeast CA. But after the early 2000s, the increasing FAP amplitude is decelerated due to the interdecadal decline of TSA accompanied by the weakened/reversed relationship between FAP and the NAHD/ENSO. Because of the corresponding breakdown of the wave train, the favorable local circulation is unavailable to support the sustained enhancement of FAP. Therefore, the multiscale coupling between the above oceanic and atmospheric modes is significantly related to the characteristic of stage of the forced aridity change in CA under the background of global warming.