We know it is getting warmer. Record warm temperatures keep coming up month after month, year after year, locally and globally. However, temperatures are but one of many indicators of global warming. With warmer temperatures come various changes that also point to a steady change in our world.
Arctic sea ice extent is diminishing
The area covered by sea ice has been smaller every year for the past 30 years, with an average decrease of 3.2% per decade. The sea ice reaches its lowest cover in September, and then grows again during the winter. Minimum cover in September is decreasing at an even faster rate of 13.4% per decade, with September 2016 seeing cover at a record low.
Exceptionally warm winter temperatures can affect the re-growth, and October 2016 – January 2017 saw the lowest daily extent of sea ice in record, likely due to record warm temperatures in the Arctic. The Arctic Report Card released in December 2016 showed a variety of records, including temperature. The volume of sea ice is also decreasing. And the trend continues in 2017.
Ocean heat content is increasing
The top half-mile of most major ocean waters is getting warmer. The amount of heat absorbed by the oceans has increased significantly over the past two decades. Warmer ocean water damages coral reefs, threatens marine ecosystems, and disrupts global fisheries. A change in ocean heat content can also alter patterns of ocean circulation, which can have far-reaching effects on global climate conditions, including changes to the outcome and pattern of meteorological events such as tropical storms, and also temperatures in the northern Atlantic region, which are strongly influenced by currents that may be substantially reduced with CO2 increase in the atmosphere.
Air temperature over ocean is increasing
Just as temperatures over land are getting warmer, so are temperatures over oceans. Warmer air near the ocean surface leads to increased evaporation, and more evaporation means more water vapor in the air. The increased amount of water vapor not only contributes to extra warming, but may feed heavy precipitation events and act as fuel for potential hurricanes, if other conditions are right.
Sea surface temperature is increasing
The surface of the ocean is getting warmer. Water temperatures measured at the surface have been higher during the past three decades than at any other time since measurements began in 1880. From 1901 through 2015, sea surface temperatures rose at an average rate of 0.13°F per decade, and they continue to rise. Warmer sea surface water can severely damage coral reefs, facilitate algal blooms, and together with warmer air temperature over the oceans, can increase the destructive potential of tropical cyclones and hurricanes.
Global sea level is rising
Records show that the sea level has been rising steadily across the globe at a rate of 0.04 to 0.1 inches per year since 1900. The current rate according to NASA is 0.13 in/year, suggesting rise is accelerating. The main reasons for the rise are thermal expansion due to higher sea water temperatures (warmer water expands) and the added water from melting of land ice, such as glaciers.
The impacts of a higher sea level can go from permanently flooded areas (below future sea level) to an increase of seasonal and “nuisance” floods, as well as worse (higher) storm surges. Sea level rise also threatens freshwater supplies and ecosystem services such as natural water filtration, as well as human coastal infrastructure.
Humidity is increasing (causing more warming)
Weather stations have recorded higher levels of water vapor in the air and when it is hot out, it feels even hotter. When surface temperature warms (both land and sea surface), it leads to more evaporation, and consequently an increase in atmospheric humidity. Because water vapor is a greenhouse gas, the increase in humidity causes additional warming.
Temperature of the lower atmosphere is increasing
The atmosphere is not warming uniformly. Warming is instead concentrated in the lower layers of the atmosphere, called the troposphere (where we live, where weather occurs, and where planes fly). Rising temperatures in the troposphere evidence the human fingerprint in global warming caused through the release of emissions from burning coal, oil, and natural gas.
Air temperature over land is increasing
Air temperature over land is increasing faster than over the oceans. The excess heat in the atmosphere has been measured by weather stations over land across the globe. The increase in air temperature is not homogeneous throughout the globe – for instance, the Arctic is seeing a faster rate of warming than other areas. The excess heat in the air can be linked to increased frequency of heat waves across the globe, wider distribution of insect pests (e.g., pine beetle in the northwest U.S.), and various health-related issues such as the spread of tropical diseases further north into the U.S.
Snow cover is reduced, and snow is melting earlier
The area covered by snow in the Northern Hemisphere in the spring is getting smaller, not only because there is less snow in some areas and years, but also because snowpacks are melting earlier due to warmer temperatures. People all over the world rely on melting snow to replenish their water supplies, and the earlier melt (and sometimes smaller accumulation) reduces the water availability for these people, threatening their way of life. In the U.S., scientists believe that the reduced snowpack and early melting of snow in the Sierra Nevada was one of the factors behind the long, extreme drought that began affecting California in 2011.
Glaciers are melting
Glaciers are an essential source of water for many places around the world, where people depend on glacial meltwater to replenish their aquifers. Glaciers are retreating at an astonishing pace – three times the ice volume stored in the entirety of the European Alps is lost globally every year. As glaciers melt, water supplies are at risk and there is also the potential for soil destabilization and erosion because glacial waters carve rivers through their melting path.
Melting glaciers also can lead to hazardous conditions such as landslides and avalanches for communities below, who also value glaciers’ aesthetic value: the sheer loss of the natural beauty of glaciers majestic sights is culturally significant. The evidence of these changes is clear: Glacier National Park only has 25 of the estimated 150 glaciers that existed there in the 1800’s.