In a 4°C hotter world, Bangladesh’s land, air, and sea will be hotter on average year-round. Global-mean near-surface air temperature is already 0.8 degrees Celsius (°C) above preindustrial (1851-79) levels. By the same metric, a rise exceeding 4°C by the year 2100 is around 85% likely in a scenario of no climate policies with comparatively high greenhouse gas emissions (Riahi et al. 2013. "Locked into Copenhagen Pledges—Implications of Short-Term Emission Targets for the Cost and Feasibility of Long-Term Climate Goals." Tech. Forecasting & Soc. Change [in press]. doi: 10.1016/j.techfore.2013.09.016).
Such a change is 40% likely under a scenario combining existing policies with cautious implementation of announced new climate policies (International Energy Agency. 2012. World Energy Outlook 2012. Paris: OECD. doi: 10.1787/weo-2012-3n). In a business-as-usual scenario, warming exceeds 2°C by the 2040s (World Bank 2013).

Sea level will rise by at least 1 meter and could rise several meters more in the future. If the ocean surface temperature rises by 2°C and the sea level rises by 1 meter, the storm surge from a cyclone with wind speed of 246 kilometers/hour could be 8.6 meters, compared with the 7.6-meter storm surge from the Bangladesh 1991 cyclone with speeds of 225 kilometers/hour (van Vliet et al. 2013. "Global River Discharge and Water Temperature under Climate Change." Global Env. Ch. 23 (2): 450–64.).
If the Greenland ice sheet irreversibly loses mass, which is plausible at 1.6°C warming, or if the West Atlantic ice sheet melts, the sea level could rise by five meters or more and pose an existential risk to Bangladesh (World Bank 2013, 161).

Seven out of 10 summers will be abnormally hot. In a 4°C Hotter world, “unusual heat” (a multimodal temperature mean more than three times the standard deviation) is 60-80% likely in Bangladesh during June, July, and August (World Bank 2013, 113, fig. 5.4).
Heat waves will break record temperatures, above 45°C. In a 4°C hotter world, Bangladesh will experience high temperatures above any levels seen in the past 100 years (World Bank 2013, 113). The Indian subcontinent including Bangladesh experienced a heat wave with temperatures of 45–50°C in 2005. (World Meteorological Association. 2006. "WMO Statement on the Status of the Global Climate in 2005.")

Northern Bangladesh will shift to a much hotter climate regime. Monthly temperature distributions will move 5-6 standard deviations toward hotter values. (World Bank 2013, 113).
Inundation will be deeper, more widespread, and will affect millions more people, especially the poor. If seas rise by 50 centimeters, a 1-in100-year coastal flood around 2070 would affect more than 11 million people in Dhaka alone (Hansen et al. 2011. "A Global Ranking of Port Cities with High Exposure to Climate Extremes." Climate Change 104 (1): 89–111). If seas rise by 1 meter, inundation could displace 13 million Bangladeshis (Huq, Ali, and Rahman. 1995. "Sea-Level Rise and Bangladesh: A Preliminary Analysis." J. Coastal Res. 14: 44–53). If seas rise by 1 meter and storminess increases, surges could affect up to 1.54 million people by 2070 in the coastal cities of Dhaka, Chittagong, and Khulna (Brecht et al. 2012. "Sea-Level Rise and Storm Surges: High Stakes for a Small Number of Developing Countries." J. Env. & Dev. 21 (1): 120–38). Areas with the highest inundation have the highest share of poor people (World Bank 2010, "Economics of Adaptation").

Food insecurity will worsen. Severe cyclones at high tide will inundate more land with more water. (World Bank. 2010. "Economics of Adaptation to Climate Change: Bangladesh", xv.) Crops production per person will be a third lower in the 2040s than in 2000. Agricultural production in Patuakhali and Khulna could fall by as much as 40%. Predicted 40% decrease in crop production would occur if sea level rises by 65 centimeters by the 2080s. (Yu et al. 2010. Climate Change Risks and Food Security in Bangladesh. Washington, DC: World Bank).

Farmers will produce 3.9% less rice each year. The economy would lose $2.9 billion each year—and more as losses mount. ( World Bank 2010, "Economics of Adaptation”.)
In a 4°C hotter world, there is a 66% chance that the sea level near Dhaka will be 100–150 centimeters higher than the 1986–2005 average, excluding local land subsidence due to natural or human influence (World Bank 2013, 117, fig. 5.8).

In a 4°C hotter world, river flows could increase as much as 50% for the Ganges, enough to meet forecast increase in demand for water (Fung, Lopez, and New. 2011. "Water Availability in +2°C and +4°C Worlds." Phil. Trans. R. Soc. A. 369 (1934): 99–116). In a 2°C hotter world, the Ganges-Brahmaputra river average high flows will 5% be higher, and low flows 13% lower by 2071-2100, relative to 1971-2000 (van Vliet at al. 2013. "Global River Discharge"). In a 1.5°C hotter world the Ganges-Brahmaputra-Meghna summer river flows will be 6–18% greater by the 2050s than they are today (World Bank 2010, "Economics of Adaptation"). Groundwater stress includes lower water stores, depletion, and salinization (World Bank 2013, 109, 119, 125).

Around 191,637 people have died from natural disasters in the last 30 years in Bangladesh, of which cyclones alone have accounted for 167,178 deaths. (Maplecroft Report)

Almost two billion people were affected by natural disasters in the last decade of the 20th century, 86% of them by floods and droughts. (WFP)
 

UNDP identifies Bangladesh to be the most vulnerable country in the world to tropical cyclones and the sixth most vulnerable country to floods. (UNDP (2004) A Global Report: Reducing Disaster Risk: A challenge for Development)

Over the last 35 years, the Government of Bangladesh has invested over $10 billion (at constant 2007 prices) to make the country more climate resilient and less vulnerable to natural disasters. (Bangladesh Climate Change Strategy and Action Plan)

In an ‘average’ year, approximately one quarter of the country is inundated. (Bangladesh Climate Change Strategy and Action Plan)

Once in every 4 to 5 years, there is a severe flood that may cover over 60% of the country and cause loss of life and substantial damage of infrastructure, housing, agriculture and livelihoods. (Bangladesh Climate Change Strategy and Action Plan)

About 830,000 ha cultivable land has been damaged by saline water intrusion from Bay of Bengal. At present 13% areas in Bagerhat, Khulna & Sathkhira, southwestern coastal districts of Bangladesh are affected by salinity which will increase to 16% by 2050 and 18% by 2100. (Climate Change and Bangladesh, Climate Change Cell, Department of Environment & Forest, Bangladesh, IPCC Report)

According to a Government estimate, around 1.5 million people took refuge in cyclone shelters when cyclone Sidr hit the coast of Bangladesh in November 2007. (Bangladesh Climate Change Strategy and Action Plan)