Greenhouse gas concentrations reached a new high in 2018, driven by human emissions from fossil fuels, land use and agriculture.
Three greenhouse gases – CO2, methane (CH4) and nitrous oxide (N2O) – are responsible for the bulk of additional heat trapped by human activities. CO2 is by far the largest factor, accounting for roughly 50% of the increase in “radiative forcing” since the year 1750. Methane accounts for 29%, while nitrous oxide accounts for around 5%. The remaining 16% comes from other factors including carbon monoxide, black carbon and halocarbons, such as CFCs.
Human emissions of greenhouse gases have increased atmospheric concentrations of CO2, methane and nitrous oxide to their highest levels in at least a few million years – if not longer. The figure below shows concentrations of these greenhouse gases – in parts per million (ppm) for CO2 and parts per billion (ppb) for methane and nitrous oxide – from the early 1980s through September 2018 (the most recent data currently available).
Global concentrations of CO2, methane (CH4) and nitrous oxide (N2O). Based on data from NOAA’s Earth Systems Research Laboratory. Chart by Carbon Brief using Highcharts.
Sea ice remains low
Sea ice spent much of early 2018 at record lows in the Arctic and quite low in the Antarctic. It recovered somewhat at both poles by mid-year, but by the end of the year had returned to record lows in the Antarctic and is currently the third lowest on record in the Arctic. The Arctic spent most of the year well below the historical range over the 1979-2010 period and saw the sixth lowest summer minimum since records began in the late 1970s
The figure below shows both Arctic and Antarctic sea ice extent in 2018 (solid red and blue lines), the historical range in the record between 1979 and 2010 (shaded areas) and the record lows (dotted black line).
Arctic and Antarctic daily sea ice extent from the US National Snow and Ice Data Center. The bold lines show daily 2018 values, the shaded area indicates the two standard deviation range in historical values between 1979 and 2010. The dotted black lines show the record lows for each pole. Chart by Carbon Brief using Highcharts.
Sea-ice extent only tells part of the story about changes at the poles; thickness (and volume) are also important variables, though they are more difficult to measure. The Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS) project provides estimates of sea-ice volume since 1979, shown in the figure below.
Graph showing Arctic sea-ice volume anomalies from 1979 through 2018 from PIOMAS.
Arctic sea-ice volume anomalies from 1979 through 2018 from PIOMAS.
Sea-ice volume shows a clear downward trend. While some individual months have lower or higher values than others, the range in 2018 to date is consistent with the long-term decline in Arctic sea-ice volume. Unfortunately, due to the government shutdown in the US, sea-ice volume estimates for December 2018 are not yet available.
Looking ahead to 2019 surface temperatures
While a modest La Niña event helped drag 2018 down to being the fourth warmest year on record, modest El Niño conditions have developed over the past few months and are expected to persist through late spring. This will help bump up 2019 temperatures, all things being equal.
Both the UK Met Office and NASA’s Dr Gavin Schmidt have already predicted what temperatures might look like in 2019. Both suggest that 2019 will most likely be warmer than 2018, with a best guess of a second place finish and a range of anywhere between the warmest year and the fifth warmest year on record.
The figure below, by Schmidt, shows 1980-2017 temperatures in black, a 2018 projection made at the end of 2017 in dark blue, a 2018 projection using data through October in light blue, and a 2019 projection based on modelled future El Niño conditions in green.
Graph showing Temperature projections for 2018 and 2019 provided by Dr Gavin Schmidt in late November 2018, using data from NASA GISTemp.
Temperature projections for 2018 and 2019 provided by Dr Gavin Schmidt in late November 2018, using data from NASA GISTemp.
Methods
Carbon Brief has produced a raw global temperature record using unadjusted ICOADS sea surface temperature measurements gridded by the UK Hadley Centre and raw land temperature measurements assembled by NOAA in version 4 of the Global Historical Climatological Network (GHCN).
Raw land temperatures were calculated by assigning each station to a 5×5 latitude/longitude grid box, converting station temperatures into anomalies relative to a 1971-2000 baseline period, averaging all the anomalies within each grid box for each month, and averaging all grid boxes for each month weighted by the land area within each grid box. Raw combined land/ocean temperatures were estimated by averaging raw land and ocean temperatures weighted by the percent of the globe covered by each. The resulting global temperature estimate was “rebaselined” to 1981-2010 to be comparable to other estimates shown.
For the plot showing temperatures without El Niño/La Niña, the effect of ENSO was removed from each surface temperature record for each month using an approach adapted from Foster and Rahmstorf (2011). A regression model was used to estimate the impact of ENSO on each group’s temperature series from January 1950 through December 2018, using a three month lagged Oceanic Niño Index. This estimated ENSO impact was then subtracted from the temperature series to calculate what the temperature records might look like in the absence of an ENSO signal.
https://www.carbonbrief.org/state-of...ld-warmed-2018
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