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Bangladesh

Explore historical and projected climate data, climate data by sector, impacts, key vulnerabilities and what adaptation measures are being taken. Explore the overview for a general context of how climate change is affecting Bangladesh.

What is Climate Change ?

Understanding the Big Picture

The 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 annually 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.

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.

Paleoclimate

Reconstructions from paleoclimate archives allow current changes in atmospheric composition, sea level and climate (including extreme events such as droughts and floods), as well as future projections, 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 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 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, shown in Figure 2.

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 means

Earth’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 Change

Observed 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 the RCPs - Future Climate Scenarios

Predicting future climate scenarios require a number of assumptions to be made about the direction of the future global climate and its sensitivity to projected emission and mitigation pathways. The Representative Concentration Pathways (RCPs) were developed by the Intergovernmental Panel on Climate Change (IPCC) and are used to make projections based on anthropogenic GHG emissions, which are driven primarily by population size, economic activity, lifestyle, energy use, land use patterns, technology and climate policy.

The RCPs describe four different 21st century pathways of GHG emissions and atmospheric concentrations, air pollutant emissions, and land use. The RCPs include a stringent mitigation scenario (RCP2.6), two intermediate scenarios (RCP4.5 and RCP6.0) and one scenario with very high GHG emissions (RCP8.5). Scenarios without additional efforts to constrain emissions (’baseline scenarios’) lead to pathways ranging between RCP6.0 and RCP8.5. The radiative forcing is measured in watts per square meter (W/m2) and 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.

RCPs should be considered as ‘what-if scenarios’ to help guide the understanding of possibility for projected change and future climates based on a specific emission pathway. It is important to note that probabilities, or the likelihood of occurrence are not associated with the RCPs. The four RCP scenarios are described below.

  • Stringent mitigation scenario (RCP2.6): A "peak-and-decline" scenario; its radiative forcing level first reaches a value of around 3.1 W/m2 by mid-century and returns to 2.6 W/m2 by 2100. In order to reach such radiative forcing levels, GHG emissions (and indirectly emissions of air pollutants) are reduced substantially over time. RCP2.6 is representative of a scenario that aims to keep global warming likely below 2°C above pre-industrial temperatures

  • Medium-low emissions scenario (RCP4.5): A stabilization scenario which assumes action is taken to curb climate change by all countries resulting in a global average temperature rise of no more than 2 ºC and 3 ºC above pre-industrial temperature levels by the year 2100.

  • Medium-high emission scenario (RCP6.0): A stabilization scenario in which total radiative forcing is stabilized shortly after 2100, without overshoot by the application of a range of technologies and strategies for reducing GHG emissions

  • High-end emissions scenario (RCP8.5): This scenario represents the extreme end of plausible climate change, delivering an estimated global average temperature increase of approximately 5-6ºC by 2100, relative to pre-industrial temperature levels. RCP8.5 is commonly recognized as ‘business as usual’.

The CMIPs

Understanding 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 science coordination and partnerships, the World Climate Research Program (WCRP) supports the scientific 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 CMIPs are comprised of multiple models undertake by the scientific community. Climate models are based on well-documented physical processes to simulate the transfer of energy and materials through the climate system. Climate models, also known as General Circulation Models (GCMs) and Earth System Models (ESMs), use mathematical equations to characterize how energy and matter interact in different parts of the ocean, atmosphere, and land and are used to simulate climate aspects including the temperature of the atmosphere and the oceans, precipitation, winds, clouds, ocean currents and sea-ice extent. The current CMIP data ensemble is in the Fifth Phase: CMIP5. The CMIPs are the foundational data for the global climate change projections presented in the IPCC Assessment Reports; the IPCC Fifth Assessment Report uses CMIP5 and the IPCC Sixth Assessment Report will use CMIP6.

The CMIP5 multi-model experiment (coordinated through WCRP) presents an unprecedented level of information on which to base assessments of climate variability and change. CMIP5 includes new ESMs in addition to Atmosphere-Ocean General Circulation Model (AOGCMs), new model experiments and more diagnostic output. CMIP5 is much more comprehensive than the preceding CMIP3 multi-model experiment. CMIP5 has more than twice as many models, many more experiments (that also include experiments to address understanding of the responses in the future climate change scenario runs), and significantly more data as compared to CMIP3. A larger number of forcing agents are treated more completely in the CMIP5 models, with respect to aerosols and land use particularly. Black carbon aerosol is now a commonly included forcing agent. Considering CO2, both ‘concentrations-driven’ projections and ‘emissions-driven’ projections are assessed from CMIP5. These allow quantification of the physical response uncertainties as well as climate–carbon cycle interactions. 

CCKP currently uses data derived from CMIP5, upon full release of processed CMIP6 data, CCKP will update as appropriate.

Individual Models vs. Multi-Model Ensembles

Climate 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, Uncertainty

Decadal, 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.

 

References

Clark, 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 (2016). 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

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

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