Environmental effects Further information: Effects of climate change on oceans and Effects of climate change on the water cycle The environmental effects of climate change are broad and far-reaching, affecting oceans, ice, and weather. Changes may occur gradually or rapidly. Evidence for these effects comes from studying climate change in the past, from modelling, and from modern observations.[154] Since the 1950s, droughts and heat waves have appeared simultaneously with increasing frequency.[155] Extremely wet or dry events within the monsoon period have increased in India and East Asia.[156] The rainfall rate and intensity of hurricanes and typhoons is likely increasing,[157] and the geographic range likely expanding poleward in response to climate warming.[158] Frequency of tropical cyclones has not increased as a result of climate change.[159] Historical sea level reconstruction and projections up to 2100 published in 2017 by the U.S. Global Change Research Program[160] Global sea level is rising as a consequence of glacial melt, melt of the Greenland ice sheets and Antarctica, and thermal expansion. Between 1993 and 2020, the rise increased over time, averaging 3.3 ± 0.3 mm per year.[161] Over the 21st century, the IPCC projects that in a very high emissions scenario the sea level could rise by 61–110 cm.[162] Increased ocean warmth is undermining and threatening to unplug Antarctic glacier outlets, risking a large melt of the ice sheet[163] and the possibility of a 2-meter sea level rise by 2100 under high emissions.[164] Climate change has led to decades of shrinking and thinning of the Arctic sea ice.[165] While ice-free summers are expected to be rare at 1.5 °C degrees of warming, they are set to occur once every three to ten years at a warming level of 2 °C.[166] Higher atmospheric CO2 concentrations have led to changes in ocean chemistry. An increase in dissolved CO2 is causing oceans to acidify.[167] In addition, oxygen levels are decreasing as oxygen is less soluble in warmer water.[168]Dead zones in the ocean, regions with very little oxygen, are expanding too.[169]