The most central measure of climate change is the increase in the global average surface temperature. Thermometer networks regularly measure temperature around the world and these measurements are combined to make a global average. While the number and quality of surface thermometers has increased with time, precise global records can be assembled stretching back to the 19th century. While there is some disagreement on the margins, research groups around the world have found that the global average surface temperature has increased by about 1 °C since the middle of the 19th century.
Annual average temperature records from 1880-2016, shown as a departure from the 1880-1920 average. The colored lines show the temperatures as estimated by four different research groups: NASA GISS (brown), NOAA (green), Hadley Centre (red), and Berkeley Earth (blue).
The average increase in global temperatures has manifested itself in a host of temperature records. Each of the last three decades was hotter than the previous decade, with the hottest decade in the instrumental record so far being 2000 to 2010. Since 2010, temperatures have reached even higher levels, with the years 2014, 2015, and 2016 each successively registering as the hottest year since monitoring global temperatures with land-based thermometers began in the mid 19th century (the so-called “instrumental record”).
For more read: Are global temperature records reliable?
During the course of this century-long warming, there have been periods where warming slowed or stopped for up to 10- to 15-year periods, but risen again such that the overall warming has continued. Explanations for these decadal variations in temperature invoke short-term changes in natural climate drivers (e.g., volcanic eruptions or solar activity), human climate drivers (e.g., changes in emissions of sulfur dioxide, a particulate pollution that has a cooling effect on climate) or random fluctuations on the part of the climate that can result from changes in the large-scale circulation of the ocean (e.g., changes in the exchange of heat between the ocean and the atmosphere resulting from the El Niño/La Niña oscillations or large scale ocean circulation in the Pacific Ocean).
For more read: Has Global Warming Stopped, Paused, or Slowed Down?
Along with temperature, sea level rise is a sensitive indicator of climate change. Warm periods in Earth’s history featured much higher sea levels, a result of ice sheets and glaciers melting and warming ocean temperatures. Over the 20th century, global average sea level increased by about 8 inches after not having changed very much over at least the last several thousand years.24 As for temperature, there is not much technical disagreement about the extent or causes of total sea level rise over the last century,25 which is the result of warming of the global ocean (which causes water to expand) and melting ice from glaciers and polar regions (adding water).
A larger body of evidence shows other components of the climate system undergoing significant changes, including the warming of the lower part of the atmosphere (known as the troposphere, which has been observed since the mid-20th century), cooling of the stratosphere26 (observed since the mid-20th century), the warming of the oceans (observed globally since the 1970s), the retreat of glaciers (observed at many glaciers since the 19th century), the retreat of snow cover (observed by satellites since the 1970s), and receding sea ice in the Arctic (observed by satellite since the 1980s).