The World Meteorological Organisation (WMO) Statement on the state of the global climate in 2018 was published on 25 November 2019.
2018 is on course to be the fourth warmest year on record.
This means that the past four years—2015, 2016, 2017 and 2018—taken together are the four warmest years on record. In contrast to the other top warmest years, 2018 began with La Niña conditions, which are typically associated with lower global temperatures.
Atmospheric carbon dioxide concentration continued to increase in 2018.
Carbon dioxide concentrations reflect a balance between emissions due to human activities and the net uptake by the biosphere and oceans. Increasing levels of carbon dioxide and other greenhouse gases in the atmosphere are key drivers of climate change.
Mean Global Mean Sea Level for the period from January to July 2018 has been 2 to 3 mm higher than for the equivalent period in 2017.
More than 90% of the energy trapped by greenhouse gases, goes into the oceans. Ocean Heat Content provides a direct measure of the energy that accumulates in the upper layers of the ocean. For each three-month period in 2018, the ocean heat content in the upper 700m and upper 2000m were either the highest or second highest on record.
In the past decade, the oceans absorbed around 25% of anthropogenic carbon dioxide emissions. Absorbed carbon dioxide reacts with seawater and changes the pH of the ocean. This process is known as ocean acidification. Observations in the open-ocean over the last 30 years have shown a clear trend of decreasing pH. The IPCC Fifth Assessment report found that there was a decrease in the surface ocean pH of 0.1 units since the start of the industrial revolution (1750). Changes in pH are linked to shifts in ocean carbonate chemistry that can affect the ability of marine organisms such as molluscs and reef-building corals, to build and maintain shells and skeletal material. This makes it particularly important to fully characterise changes in ocean carbonate chemistry.
Arctic sea-ice extent was well below average throughout 2018 and was at record-low levels for the first two months of the year. The annual maximum occurred in mid-March and the March monthly extent was 14.48 million square km, third lowest on record and approximately 7% below the 1981-2010 average. The Arctic sea-ice extent reached its minimum in mid-September. The September monthly sea-ice extent was 5.45 million square km, approximately 28% below average. This ranked as the 6th smallest September extent on record. The 12 smallest September extents have all occurred in the 12 years since 2007.
Antarctic sea-ice extent was also well below average throughout 2018. The annual minimum extent occurred in late February and the monthly average was 2.28 million square km, 33% below average and ranked record low in the C3S dataset and 2nd lowest in the NSDIC data. For the months February through August, the monthly extent ranked among the ten smallest on record. The Antarctic sea-ice extent reached its annual maximum extent in late-September and early-October. The September monthly average extent was 17.82 million square km, 4% below average and ranked within the 5th smallest.
In the hydrological year 2016/17, observed glaciers experienced an ice loss of 0.850 meter water equivalent (m w.e.). Preliminary estimates for 2017/18 indicate a similarly negative mass balance year with an ice loss of 0.7 m w.e. With this, seven out of the ten most negative mass balance years were recorded after 2010.