Empirical evidence alone—without reference to climate models—indicates that a general warming trend is affecting weather and ecosystems with increasing impacts on humans. Recent weather has been characterized by an increase in the frequency of extreme weather events—floods, droughts, tornadoes, glacial lake outbreaks, extreme coastal high-water levels, heat waves, cold spells, etc—and this will continue during the next 20 years.
According to the recent IPCC Special Report on Extreme Events (SREX), climate and socioeconomic trends will reinforce extreme weather, making it more frequent and intense. Although the number of tropical and extratropical cyclones probably will not increase, the average maximum wind speed for tropical cyclones will increase. Meanwhile, population growth and economic development will widen the exposure of people and property. The key unknown is whether improved disaster risk management measures will be adopted to effectively cope with these changing conditions by 2030.
Food security has been aggravated partly because during the last two decades the world’s land masses are experiencing weather conditions outside of expected norms. Observed temperature increases (though enhanced in the Arctic) are not solely a high-latitude phenomenon. Recent scientific work shows that temperature anomalies during growing seasons and droughts have lessened agricultural productivity. Degraded agriculture productivity, when coupled with more protectionist national policies tightening global supply, undercuts food security, especially in impoverished regions.
Flows in the Nile, Tigris-Euphrates, Niger, Amazon, and Mekong river basins have been diminished by droughts that have been persistent over the past decade. These trends are consistent with the expected effects of increased greenhouse gas (GHG) concentrations in the atmosphere, but due to the limited observational record (60 years) and a lack of understanding of decadal variability, one cannot discount the possibility that observed trends are due to other natural causes of weather variability.
Dramatic and unforeseen changes are occurring at a faster rate than expected in regions with frozen water. Current estimates suggest that Arctic summer sea ice will vanish in the period 2030-2050. Changes are occurring in the major ice shelves (Greenland and Antarctica) that were unforeseen even five years ago. Future rates of change are currently unpredictable because observed changes have outpaced the development of ice-prediction models. Scientists now estimate sea-level rise (SLR) of one meter or greater by the end of the century, most of which is expected to occur toward the end of the century. Sea-level rise could increase with rapid melt of either the Greenland Ice Sheet or the West Antarctica Ice Shelf. In the next 20 years, barring collapse of the ice shelves, the SLR trend will be modest and consistent with the recent record, about 3.3±0.4mm/year (that is, an additional ~2.5 inches global average sea-level rise). However, even this change, when coupled with potential storm surges from more intense storms and subsidence of delta lands, will have a significant adverse impact on coastal regions and Pacific small-island states.
Improved understanding of the changes in the stratosphere reveal that the ozone layer over the northern hemisphere is diminishing, leading to the possibility of greater ultraviolet (UV) radiation over northern hemisphere countries. Based on a better understanding of climate sensitivity and emissions, the present emissions pathway will lead to approximately 2°C warming by mid-century and approximately 3° to 6°C by end of century, depending on economic performance, technological advances, and energy policy. By 2030 the emissions trajectory will be cast, determining this century’s climate outcome.