Global warming: Temperature evolution

 (Annual (thin lines) and five-year lowess smooth (thick lines) for the temperature anomalies averaged over the Earth’s land area and sea surface temperature anomalies (blue line) averaged over the part of the ocean that is free of ice at all times (open ocean).)

Terrestrial temperature measurements carried out during the twentieth century show a rise in temperature on most of the Earth’s surface. The elevation of this temperature is observed by measuring in various points of the globe the difference between the current temperature and a temperature taken as reference (most often the average temperature over a given period at the considered point), then by calculating the weighted average of these differences. It can be observed that the average of these differences tends to increase during the last decades. In the remainder of the article, this observation will be indicated by the periphrasis increase of the average temperature.

The observed warming occurred in two phases, the first from 1910 to 1945, the second from 1976 to today. These two phases are separated by a period of slight cooling. This global warming seems to be correlated with a strong increase in the atmosphere of the concentration of several greenhouse gases, including carbon dioxide, methane and nitrous oxide.

The ten hottest years between 1880 and 2015 based on measurements of weather stations on the ground and on the surface of the oceans
(Deviations from the 1951-1980 average)

	Years	ground stations + oceans Deviations/average	ground stations only Deviations/average
1	2015	+0,87 °C	+1,01 °C
2	2014	+0,74 °C	+0,95 °C
3	2010	+0,72 °C	+0,90 °C
4	2005	+0,69 °C	+0,89 °C
5	2007	+0,66 °C	+0,88 °C
6	2013	+0,65 °C	+0,85 °C
7	2009	+0,64 °C	+0,84 °C
8	2012	+0,63 °C	+0,82 °C
9	2006	+0,63 °C	+0,81 °C
10	1998	+0,63 °C	+0,80 °C

(Differences in temperature declined by country 1900 – 2016 (in comparison with the average temperature 1951-1980) Animation created from GISTEMP data.)

According to the 2013 IPCC assessment, the average global surface temperature rise between 1901 and 2012 is estimated at 0.9 °C (with a 90% probability that warming will be between 0.69 °C and 1.08 °C), and the first decade of the 21st century was the warmest decade since the introduction of modern temperature readings and each of the last three decades (from 1980 to 2010) was successively warmer on the surface of the Earth than all previous decades since 1850.

Global warming continued unabated from 1980 to 2010, over thirty years, a time interval generally considered by meteorologists to be sufficient as a short-term trend.

According to NASA’s Goddard Institute for Space Studies (GISS), 2015 was the hottest year since statistics began (1880), according to convergent analyzes by NASA and National scientists. Oceanic and Atmospheric Administration (NOAA), with 94% statistical certainty; it was 0.13 °C above the previous record set in 2014; 15 of the 16 warmest years recorded were after 2000, the exception being 1998. Since 1880, the average temperature on the surface of the Earth has warmed by 1.0 °C. The El Niño phenomenon contributed to this warming over most of 2015.

The IPCC’s Fourth Report finds “very likely” that the average temperatures in the northern hemisphere were higher in the second half of the twentieth century than in any other period of fifty years in the last five centuries and “probable” that they have been the highest for at least 1,300 years.

Rainfall

According to the fourth IPCC report, the distribution of rainfall has changed during the twentieth century. In particular, precipitation is reported to have risen sharply in eastern North and South America, northern Europe and northern and central Asia, while decreasing in the Sahel, Mediterranean, Southern Africa and parts of South Asia. Other experts, however, consider the current data too infrequent and incomplete for a trend of increasing or decreasing precipitation to emerge in areas of this magnitude. Since 1988, there has been a noticeable decrease in spring snow cover at mid-latitudes in the northern hemisphere. This decrease is worrying because this snow cover contributes to soil moisture and water resources.