Understanding the Big PictureThe Earth’s climate is changing and the global climate is projected to continue to change over this century and beyond. The magnitude of climate change beyond the next few decades will depend primarily on the amount of greenhouse (heat-trapping) gases emitted globally and on the remaining uncertainty in the sensitivity of the Earth’s climate to those emissions. With significant reductions in the emissions of greenhouse gases (GHGs), global annual averaged temperature rise could be limited to 2°C or less. However, without major reductions in these emissions, the increase in annual average global temperatures, relative to preindustrial times, could reach 5°C or more by the end of this century. Show
The global climate continues to change rapidly compared to the pace of the natural variations in climate that have occurred throughout Earth’s history. Trends in globally averaged temperature, sea level rise, upper-ocean heat content, land-based ice melt, arctic sea ice, depth of seasonal permafrost thaw, and other climate variables provide consistent evidence of a warming planet. These observed trends are robust and confirmed by multiple, independent research groups around the world. Observations of the climate system are based on direct physical and biogeochemical measurements, and remote sensing from ground stations and satellites. Information derived from paleoclimate archives provides a long-term context of past climates. Different types of environmental evidence are used to understand what the Earth’s past climate was like and why. Records of historical climate conditions are preserved in tree rings, locked in the skeletons of tropical coral reefs, sealed in glaciers and ice caps, and buried in laminated sediments from lakes and the ocean. Scientists can use those environmental recorders to estimate past conditions, extending our understanding of climate back hundreds to millions of years. Global-scale observations from the instrumental era began in the mid-19th century, and paleoclimate reconstructions extend the record of some quantities back hundreds to millions of years. Together, this provides a comprehensive view of the variability and long-term changes in the atmosphere, the ocean, the cryosphere and at the land surface. PaleoclimateReconstructions from paleoclimate archives allow current changes in atmospheric composition, sea level and climate systems (including extreme events such as droughts and floods), as well as projections of future climates, to be placed in a broader perspective of past climate variability. Past climate information also documents the behavior of slow components of the climate system including the carbon cycle, ice sheets and the deep ocean for which instrumental records are short compared to their characteristic time scales of responses to perturbations, thus informing on mechanisms of abrupt and irreversible changes. Climate records over past centuries and millennia indicate that average temperatures in recent decades over much of the world have been much higher, and have risen faster during this time period, than at any time for which the historical global distribution of surface temperatures can be reconstructed. Paleoclimate can help us understand climate change on a geological timescale rather than a few human generations. Figure 1 presents paleoclimate reconstruction for the Northern Hemisphere(NH), which reveals average annual temperatures, for the period 1983–2012 was very likely the warmest 30-year period of the last 800 years and likely the warmest 30-year period of the last 1400 years. a) shows the radiative forcing due to volcanic, solar and well-mixed greenhouse gases (WMGHGs). Different colors illustrate the two existing data sets for volcanic forcing and four estimates of solar forcing and the grey line represents WMGHGs for the period 850-2000. b) represents the simulated (red) and reconstructed (shading) Northern Hemisphere temperature anomalies. The thick red line depicts the multi-model mean while the thin red lines show the multi-model 90% range. The overlap of reconstructed temperatures is shown by grey shading. Figure 1. a) Radiative forcing (W/m2) due to volcanic, solar and well-mixed greenhouse gases for the period 850-2000. b) Reconstructed (grey) and simulated (red) Northern Hemisphere Temperature Anomalies for the period 850-2000. Model projections (Figure 2) indicate that twenty-first century global average warming will substantially exceed the Last Glacial Maximum period and even the warmest Holocene conditions; producing a climate state not previously experienced. Figure 2. Model-simulated global temperature anomalies for the Last Glacial Maximum (21,000 years ago), the mid-Holocene (6,000 years ago), and projection for 2071–2095, under RCP8.5 What this meansEarth’s climate is now changing faster than at any point in the known history of the climate, primarily as a result of human activities. There is scientific consensus that unmitigated carbon emissions will lead to global warming of at least several degrees Celsius by 2100, resulting in high-impacts of local, regional and global risks to human society and natural ecosystems. Global climate change has already resulted in a wide range of impacts across every region of the earth as well as many economic sectors. Impacts related to climate change are evident across regions and in many sectors important to society, such as human health, agriculture and food security, water supply, transportation, energy, and biodiversity and ecosystems; impacts are expected to become increasingly disruptive in the coming decades. There is very high confidence that the frequency and intensity of extreme heat and heavy precipitation events are increasing in most continental regions of the world. These trends are consistent with expected physical responses to a warming climate. The frequency and intensity of extreme high temperature events are virtually certain to increase in the future as global temperature increases. There is high confidence that extreme precipitation events will very likely continue to increase in frequency and intensity throughout most of the world. Observed and projected trends for other types of extreme events, such as floods, droughts, and severe storms, have more variable regional characteristics. What is Climate ChangeObserved changes over the 20th century include increases in global air and ocean temperature, rising global sea levels, long-term sustained widespread reduction of snow and ice cover, and changes in atmospheric and ocean circulation as well as regional weather patterns, which influence seasonal rainfall conditions. These changes are caused by extra heat in the climate system due to the addition of greenhouse gases to the atmosphere. These additional greenhouse gases are primarily input by human activities such as the burning of fossil fuels (coal, oil, and natural gas), deforestation, agriculture, and land-use changes. These activities increase the amount of ‘heat-trapping’ greenhouse gases in the atmosphere. The pattern of observed changes in the climate system is consistent with an increased greenhouse effect. Other climatic influences such as volcanoes, the sun and natural variability cannot alone explain the timing and extent of the observed changes. Climate, refers to the long-term regional or global average of temperature, humidity and rainfall patterns over seasons, years or decades. While the weather can change in just a few hours, climate changes over longer timeframes. Climate change is the significant variation of average weather conditions becoming, for example, warmer, wetter, or drier—over several decades or longer. It is the longer-term trend that differentiates climate change from natural weather variability. Human activity leads to change in the atmospheric composition either directly (via emissions of gases or particles) or indirectly (via atmospheric chemistry). Anthropogenic emissions have driven the changes in WMGHG concentrations during the Industrial Era. Radiative forcing (RF) is a measure of the net change in the energy balance of the Earth system in response to some external perturbation; positive RF leads to a warming and negative RF to a cooling. The RF concept is valuable for comparing the influence on global mean surface temperature of most individual agents affecting the Earth’s radiation balance. Figure 3 shows the Radiative Forcing and Effective Radiative Forcing (ERF), by concentration change, between 1750 and 2011, with associated uncertainty range. Figure 3. Radiative Forcing (RF) and Effective Radiative Forcing (ERF) of climate change during the Industrial Era, 1750-2011. Solid bars are ERF, hatched bars are RF, green diamonds and associated uncertainties are for RF. Figure 4. Total annual anthropogenic greenhouse gas (GHG) emissions (gigatonne of CO2-equivalent per year, GtCO2-eq/yr) for the period 1970 to 2010, by gases. Figure 4. Total annual anthropogenic GHG emissions by gases for the period, 1970-2010. Gas: CO2 from fossil fuel combustion and industrial processes; CO2 from Forestry and Other Land Use (FOLU); methane (CH4); nitrous oxide (N2O); fluorinated gases covered under the Kyoto Protocol (F-gases). Understanding Future Climate ScenariosUnderstanding our current and future climate are questions that are too large and too complex to be tackled by a single country, agency or scientific discipline. Through international scientific cooperation and partnerships, the World Climate Research Program(WCRP) supports the coordination for the production of global and regional climate model compilations, which advance our understanding of the multi-scale dynamic interactions between natural and social systems that affect climate. These efforts produce the Coupled Model Inter-comparison Projects, or CMIPs. The climate science community relies on models to understand the Earth’s carbon cycle feedbacks in response to anthropogenic emissions, which lead to changes in atmospheric concentrations of greenhouse gases and aerosol, and thus ultimately result in radiative forcings that drive the climate system changes. The CMIPs provide a coordinating framework for these studies by defining a suite of model experiments for coupled atmosphere-ocean general circulation and Earth system models. Next to more process-oriented studies, one suite of experiments under CMIP is always focused on the climate response to different plausible future societal development storylines and associated contrasting emission pathways (scenarios). The goal of these ‘scenarios’ is to outline how future emissions and land use changes could translate into responses in the climate system. While independent of the regularly produced IPCC-UNFCCC Assessment Reports, CMIP results nevertheless are coordinated and directly inform the Assessments. CMIP phase 5 (CMIP5) provided the foundation for the 5th Assessment Report released in 2013 and 2014, and the 6th Assessment Report released in 2021 and 2022, is drawing from CMIP6, the latest collection of simulations done by the climate science community around the world. The scenario approach is used to characterize the range of plausible climate futures and to illustrate the consequences of different pathways (policy choices, technological changes, etc). They are chosen to span a wide range without any tie to likelihood; the scenarios serve as ‘what if’ cases. Over the past three decades, the approach to formulating the different ‘scenarios’ has evolved from a climate-centric to an increasingly societal development-centric concept, albeit with the same underlying goal of providing insight into a range of plausible climate outcomes. To distinguish the magnitude of climate forcing, the numbering reflects a designated amount of radiative forcing measured in watts per square meter (W/m2) reached by 2100 (i.e., 2.6, 4.5, 6.0 and 8.5 W/m2 of change over pre-industrial, respectively). CMIP6 introduces 1.9 W/m2 to offer insight into the climate response that might be reflective of the Paris-Accord target. The CMIP model results, as driven by scenarios, have become standard reference inputs for work concerning climate change science, impacts, vulnerability, adaptation, and mitigation. Scenarios should be used as tools to help understand the characteristics and magnitude of emerging climate signals to inform decisions. Focusing solely on end-of-century outcomes is an inadequate way to evaluate the usefulness of a given scenario. For purposes of informing societal decisions, shorter time horizons are highly relevant. CMIP5The Representative Concentration Pathways (RCPs), presented in CMIP5, describe four different 21st century pathways. The RCPs include a stringent mitigation scenario (RCP2.6), two intermediate scenarios (RCP4.5 and RCP6.0) and one scenario with high GHG emissions (RCP8.5). Scenarios without additional efforts to constrain emissions (’baseline scenarios’) lead to pathways ranging between RCP6.0 and RCP8.5. Each RCP shows the planet trapping progressively higher amounts of energy from RCP2.6 (the lowest) to RCP8.5 (the highest). Figure 5 shows the GHG emission pathways for each RCP through to the end of the century. Figure 5. GHG Emission Pathways for each RCP from 2000-2100. RCP scenarios are described below.
CMIP6The associated socio-economic narratives for each RCP scenario are called the Shared Socioeconomic Pathways (SSPs), which have been introduced in CMIP6. They represent possible societal development and policy paths for meeting designated radiative forcing by the end of the century. CMIP6 includes scenarios with high and very high GHG emissions (SSP3-7.0 and SSP5-8.5) and CO2 emissions that roughly double from current levels by 2100 and 2050, respectively, scenarios with intermediate GHG emissions (SSP2-4.5) and CO2 emissions remaining around current levels until the middle of the century, and scenarios with very low and low GHG emissions and CO2 emissions declining to net zero around or after 2050, followed by varying levels of net negative CO2 emissions (SSP1-1.9 and SSP1-2.6). Emissions vary between scenarios depending on socio-economic assumptions, levels of climate change mitigation and, for aerosols and non-methane ozone precursors, air pollution controls. Alternative assumptions may result in similar emissions and climate responses, but the socio-economic assumptions and the feasibility or likelihood of individual scenarios are not part of the assessment. Figure 6 presents future emissions and additional warming causes for each of the SSPs. Figure 6. a) presents the annual anthropogenic (human-caused) emissions over the 2015–2100 period. Shown are emissions trajectories for carbon dioxide (CO2) from all sectors (GtCO2/yr) (left graph) and for a subset of three key non-CO2 drivers considered in the scenarios: methane (CH4, MtCH4/yr); nitrous oxide (N2O, MtN2O/yr); and sulphur dioxide (SO2, MtSO2/yr), contributing to anthropogenic aerosols in panel (b). b) demonstrates the change in global surface temperature (°C) in 2081–2100 relative to 1850–1900 given the warming contributions by groups of anthropogenic drivers and by scenario, with indication of the observed warming to date. Bars and whiskers represent median values and the very likely range, respectively. Within each scenario bar plot, the bars represent: total global warming (°C); warming contributions from changes in CO2; non-CO2 greenhouse gases and net cooling from other anthropogenic drivers (‘aerosols and land use’ bar). Narrative descriptions for the Shared Socioeconomic Pathways: SSP1 “Sustainability” (Low challenges to mitigation and
adaptation) SSP2 “Middle of the Road” (Medium challenges to mitigation and adaptation) SSP3 “Regional Rivalry” (High challenges to mitigation and adaptation) SSP5 “Fossil-fueled Development” (High challenges to mitigation,
low challenges to adaptation) For a complete description of SSP Narratives, see O'Neill et al. 2017 Individual Models vs. Multi-Model EnsemblesClimate models are mathematical representations of processes important in the Earth’s climate system. When a climate model is run it produces a 'simulation' of future climate. Multiple simulations form an ensemble. A multi-model ensemble (MME) therefore is a large number of climate model simulations. CCKP prioritizes use of MMEs for its projections as multi-model ensembles are more robust and proven to be most successful in representing the range of expected changes. Differences between the spatial structure of the data and the structure of the reality it represents must also be understood and considered in order to adequately model the impact of spatial uncertainty on model applications. While, individual models are noisier, on occasion they may better reflect the range of variability compared to the multi-model ensemble that is generally too smooth. Individual models can also have systematic biases that present themselves as strong outliers. A comparison with the multi-model ensemble is helpful to identify these potential biases and outliers. Variability, Trends, UncertaintyDecadal, inter-annual, and inter-seasonal variability exists across the climate system. Internal variability can diminish the relevance of trends over periods as short as 10 to 15 years from long-term climate change. A critical effort of projecting climate change is to understand if ‘change’ is part of the natural variability or if projected change reveals trends that are statistically significant from natural variability. Due to this, natural variability trends based on short records are very sensitive to the beginning and end dates and do not, in general, reflect longer-term climate trends. Uncertainty exists for any future projection. While advances continue to be made in the understanding of climate physics and the response of the climate system to increases in greenhouse gases, many uncertainties are likely to persist. The rate of future global warming depends on future emissions, feedback processes that dampen or reinforce disturbances to the climate system, and unpredictable natural influences on climate, like volcanic eruptions. Uncertain processes that will affect how fast the world warms for a given emissions pathway are dominated by cloud formation, but also include water vapor and ice feedbacks, ocean circulation changes, and natural cycles of greenhouse gases. Although information from past climate changes largely corroborates model calculations, this is also can have a degree of uncertainty due to potentially important factors about which we have incomplete information. ReferencesClark, P., Shakun, J., Marcott, S. et al., 2016: Consequences of twenty-first-century policy for multi-millennial climate and sea-level change. Nature Clim Change 6, 360–369. DOI:https://doi.org/10.1038/nclimate2923 IPCC, 2013: Climate Change 2013: Technical Summary. The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. URL:https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_TS_FINAL.pdf IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp. URL: https://ar5-syr.ipcc.ch/ipcc/ipcc/resources/pdf/IPCC_SynthesisReport.pdf IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. In Press. URL:https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM_final.pdf International Institute for Applied Systems Analysis (IIASA), 2014: Representative Concentration Pathways Database. URL: https://iiasa.ac.at/web/home/research/researchPrograms/TransitionstoNewTechnologies/RCP.en.html Kriegler, E., Edmonds, J., Hallegatte, S., et al., 2014: A new scenario framework for climate change research: the concept of shared climate policy assumptions, Climatic Change 122:401–414. DOI:doi:10.1007/s10584-013-0971-5 O'Neill, B., Tebaldi, C., Van Vuuren, D., et al., 2016: The Scenario Model Intercomparison Project (ScenarioMIP) for CMIP6, Geoscience Model Development 9, 3461–3482. DOI:doi:10.5194/gmd-9-3461-2016 O'Neill, B., Kriegler, E., Ebi, K. et al., 2017: The roads ahead: Narratives for shared socioeconomic pathways describing world futures in the 21st century. Global Environmental Change 42, 169-180. DOI:https://doi.org/10.1016/j.gloenvcha.2015.01.004 O'Neill, B., Carter, T., Ebi, K., et al., 2020: Achievements and needs for the climate change scenario framework. Nature Climate Change 10, 1074-1084. DOI:https://doi.org/10.1038/s41558-020-00952-0 Riahi, K. van Vuuren, D., Kriegler, E., et al. 2017: The Shared Socioeconomic Pathways and their energy, land use, and greenhouse gas emissions implications: An overview, Global Environmental Change 42, 153–168. DOI:doi:10.1016/j.gloenvcha.2016.05.009 Van Vuuren, D., Edmonds, J. Kainuma, M., et al., 2011: The representative concentration pathways: an overview, Climatic Change volume 109, Article number: 5. DOI: doi:10.1007/s10584-011-0148-z World Climate Research Program (WCRP), 2021: WCRP Coupled Model Intercomparison Project (CMIP). URL: https://www.wcrp-climate.org/wgcm-cmip World Climate Research Program (WCRP), 2021: PMIP – Paleoclimate Modeling Intercomparison Project. URL: https://www.wcrp-climate.org/modelling-wgcm-mip-catalogue/cmip6-endorsed-mips-article/1064-modelling-cmip6-pmip How does global warming occur?Global warming is the long-term heating of Earth's surface observed since the pre-industrial period (between 1850 and 1900) due to human activities, primarily fossil fuel burning, which increases heat-trapping greenhouse gas levels in Earth's atmosphere.
What is global warming short answer?Global warming is the long-term warming of the planet's overall temperature. Though this warming trend has been going on for a long time, its pace has significantly increased in the last hundred years due to the burning of fossil fuels. As the human population has increased, so has the volume of fossil fuels burned.
How does global warming affect temperature?One of the most immediate and obvious consequences of global warming is the increase in temperatures around the world. The average global temperature has increased by about 1.4 degrees Fahrenheit (0.8 degrees Celsius) over the past 100 years, according to the National Oceanic and Atmospheric Administration (NOAA).
What does the world health Organization predict will happen to the global risk of negative health outcomes?What does the World Health Organization predict will happen to the global risk of negative health outcomes (e.g, disease, infection, and death) due to global warming by 2030? The global risk of negative health outcomes will increase.
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