"General overview 2021
This report has its main focus on observations and not on the output of numerical models, with the exception of Figure 39 (see p. 38). References and data sources are listed at the end of the report.
Air temperatures
Air temperatures measured near the planet’s surface (surface air temperatures) are at the centre of many climate discussions, but the significance of any short-term warming or cooling should not be overstated.
Whenever the Earth experiences warm El Niño or cold La Niña episodes, major heat exchanges take place between the Pacific Ocean and the atmosphere above, eventually showing up as a signal in the global air temperature. However, such heat exchanges may chiefly reflect redistribution of energy between ocean and atmosphere, and not a change in the heat content of the atmosphere–ocean system.
Evaluating the dynamics of ocean temperatures is therefore just as important as evaluating changes in surface air temperatures.
Considering surface air temperature records since the 19th century, 2021 was a warm year, but cooler than most years since 2016. A moderate La Niña episode played out during 2021, underlining the importance of ocean–atmosphere exchanges.
Many Arctic regions experienced record high air temperatures in 2016, but since then, including in 2021, conditions have generally moved toward somewhat cooler conditions. The temperature peak in high northern latitudes in 2016 may have been affected by ocean heat released from the Pacific Ocean during the strong 2015–16 El Niño and subsequently transported towards the Arctic region.
This underscores how air temperatures may be affected, not only by variations in local conditions, but also by variations playing out in geographically remote regions.
Many figures in this report focus on the period since 1979 – the satellite era – when access to a wide range of observations with nearly global coverage, including temperature, became commonplace. These data provide a detailed view into temperature changes over time at different altitudes in the atmosphere.
Among other phenomena, these observations reveal that a Stratospheric temperature plateau has prevailed since 1995. Since 1979, lower Troposphere temperatures have increased over both land and oceans, but most clearly over the land. The most straightforward explanation for this is that much of the warming is caused by solar insolation, but there may be several secondary reasons, such as changes in cloud cover and land use.
Oceans
The Argo Program, which uses robotic floats to monitor ocean temperatures around the globe, and at different depths, has now achieved 18 years of global coverage, growing from a relatively sparse array of 1000 floats in 2004 to more than 3900 in December 2021. Since 2004, these floats have provided a unique ocean temperature dataset for depths down to 1900m. The data is currently updated to August 2020.
Although the oceans are much deeper than 1900m, and the published Argo data series still is relatively short, interesting features are now emerging from these observations.
For example, since 2004, the upper 1900m of the oceans have experienced a globally averaged net warming of about 0.07°C. The maximum net warming (about 0.2°C) affects the uppermost 100 m, mainly near the Equator, where the greatest amount of solar radiation is received.
At greater depths, a small (about 0.025°C) net warming has occurred between 2004 and 2020, according to the Argo floats. Warming is seen across the Equatorial oceans, which, due to the spherical form of the planet, represent a huge surface area.
Simultaneously, the northern oceans (55– 65°N) have experienced a marked cooling at depths down to 1400 m, and slight warming be- low that. The southern oceans (55–65°S) have warmed slightly at most depths since 2004, but mainly near the surface. However, as discussed later in this report, averages may be misleading, and better insight is often gained by looking at the details.
Sea level
Global sea level is monitored by satellite al-timetry and by direct measurement using tide gauges. While the satellite record suggests a global sea-level rise of about 3.3 mm per year or more, data from tide gauges all over the world suggest a stable rise of 1–2mm per year. The tide gauges indicate no recent acceleration (or deceleration) of sea-level rise.
The marked difference (a ratio of about 1:2) between the two datasets has no universally accepted explanation, but it is known that the satellite observations face complications in areas near the coast (see, for example, Vignudelli et al. 2019). Whatever the truth, the tide-gauge data are more relevant for local coastal planning purposes.
Sea ice
In 2021, global sea-ice cover remained well below the average for the satellite era, but is now increasing. At the end of 2016, it reached a marked minimum, at least partly caused by the operation of two different natural variation patterns, affecting sea ice in the Northern and the Southern Hemisphere, respectively.
These variations had simultaneous minima in 2016. The trend towards stable or higher ice extent at both poles probably began in 2018, and has since strengthened. The marked Antarctic 2016 sea-ice reduction was the result of unusual wind conditions.
Snow cover
Variations in global snow cover are mainly caused by changes in the Northern Hemisphere, where all the major land areas are located. Southern Hemisphere snow is mainly found in the Antarctic, and cover is therefore relatively stable.
The Northern Hemisphere average snow cover has been stable since the onset of satellite observations, although local and regional interannual variations may be large.
Since 1979, Northern Hemisphere snow cover in autumn has been slightly increasing, the mid-winter cover is basically stable, and the spring cover is slightly decreasing. In 2021, the Northern Hemisphere seasonal snow cover was close to the 1972–2020 average.
Storms and hurricanes.
Hurricanes
The most recent data on global tropical storm and hurricane accumulated cyclone energy (ACE) is well within the range observed since 1970. In fact, the ACE data is highly variable over time, with a significant 3.6-year variation, but without any clear trend towards higher or lower values.
A longer series for the Atlantic Basin, however, indicates there may be an oscillation of about 60 years’ duration for tropical storm and hurricane ACE. The number of hurricanes making landfall in the continental United States remains within the range for the entire observation period since 1851. ... "