The climate data for 2018 is now mostly in

[MissBehavin]

The climate data for 2018 is now mostly in, though the ongoing shutdown of the US government has caused some datasets to be delayed.

In this article, Carbon Brief explains why last year proved to be so remarkable across the oceans, atmosphere, cryosphere and surface temperature of the planet.

A number of records for the Earth’s climate were set in 2018:

It was the warmest year on record for ocean heat content, which increased markedly between 2017 and 2018.
It was the fourth warmest year on record for surface temperature.
It was the sixth warmest year in the lower troposphere – the lower part of the atmosphere.
Greenhouse gas concentrations reached record levels for CO2, methane, and nitrous oxide.
Sea ice was well below the long-term average at both poles for most of the year. The summer Arctic sea ice minimum was the sixth lowest since records began in the late 1970s.
Warmest year on record in the oceans
Last year was the warmest on record for the heat content of the world’s oceans. Ocean heat content (OHC) has increased by around 370 zettajoules – a billion trillion joules – since 1955. The heat increase in 2018 alone compared to 2017 – about 9 zettajoules – is around 18 times more than the total energy used by everyone on Earth in 2018.

Human-emitted greenhouse gases trap extra heat in the atmosphere. While some of this warms the Earth’s surface, the vast majority – around of 93% – goes into the oceans. About two thirds of this accumulates in the top 700 metres, but some also ends up in the deep oceans. Annual OHC estimates between 1955 and present for both the upper 700m and 700m-2000m depths of the ocean are shown in the figure below.

n many ways, OHC represents a much better measure of climate change than global average surface temperatures. It is where most of the extra heat ends up and is much less variable on a year-to-year basis than surface temperatures.

Changes in the amount or rate of warming are much easier to detect in the OHC record than on the surface. For example, OHC shows little evidence of the slowdown in warming in the mid-2000s. It also shows a distinct acceleration after 1991, matching the increased rate of greenhouse gas emissions over the past few decades.

Just about every year since 1991 has set a new OHC record, showing that heat has continued to accumulate in the Earth system as concentrations of atmospheric greenhouse gases have increased.

As Carbon Brief discussed in a recent guest post, estimates of OHC published over the past few years represent a large upward revision of past estimates. They also now agree with projections made by global climate models.

Fourth warmest year on the surface
Global surface temperatures in 2018 were the fourth warmest on record since 1850, when global temperatures can first be calculated with reasonable accuracy. Temperatures in 2018 were between 0.9C and 1.1C warmer than temperatures in the late 19th century (between 1880 and 1900), depending on the temperature record chosen. Temperatures were dragged down a bit by a modest La Niña event earlier in the year, while the currently emerging modest El Niño event will mainly impact temperatures in 2019.

Global surface temperatures are reported by a number of different international groups, including NASA, NOAA, Met Office Hadley Centre/UEA, Berkeley Earth and Cowtan and Way. Copernicus/ECMWF also produces a surface temperature estimate based on a combination of measurements and a weather model – an approach known as “reanalysis”.

These records are created by combining ship- and buoy-based measurements of ocean sea surface temperatures with temperature readings of the surface air temperature from weather stations on land.

The chart below compares the annual global surface temperatures from these different groups since 1850, though some records – NASA and NOAA – do not start until 1880, and the Copernicus/ECMWF reanalysis dataset begins in 1979. Values are shown relative to a common baseline period, the 1981-2010 average temperature for each series. [Click the figure legend to show or hide different temperature records.]



Annual global mean surface temperatures from NASA GISTemp, NOAA GlobalTemp, Hadley/UEA HadCRUT4, Berkeley Earth, Cowtan and Way, Copernicus/ECMWF and Carbon Brief’s raw temperature record (see below). Anomalies plotted with respect to a 1981-2010 baseline. Chart by Carbon Brief using Highcharts.
The global warming seen is not due to any adjustments made to the underlying temperature records. The figure above includes a “raw records” line (shown as a dotted line) calculated by Carbon Brief using data not subject to any adjustments or corrections for changes in measurement techniques. These adjustments make little difference to the record after 1950. Before then, the adjusted temperature records actually show less warming than the raw data.

The figure below shows surface temperatures since 1970, a period during which atmospheric greenhouse gas concentrations have been rapidly increasing. While 2018 is not quite as warm as the past three years, it is in-line with the longer-term warming trend seen in the data.



Annual global mean surface temperatures from NASA GISTemp, NOAA GlobalTemp, Hadley/UEA HadCRUT4, Berkeley Earth, Cowtan and Way, Copernicus/ECMWF and Carbon Brief’s raw temperature record (see below). Anomalies plotted with respect to a 1981-2010 baseline. Chart by Carbon Brief using Highcharts.
Short-term variability in temperature records is mostly due to the influence of El Niño and La Niña events, which have a short-term warming or cooling impact on the climate. Other dips are associated with large volcanic eruptions. The longer-term warming of the climate is due to increases in atmospheric CO2 and other greenhouse gases emitted from human activity.

To assess the effects of El Niño and La Niña on the surface temperature record, Carbon Brief has produced an estimate of what temperatures would be in the absence of these events. The figure below shows estimated temperatures with El Niño removed; see the methods section at the end of the article for details on the approach used. The Copernicus records is not included in the figure, as it does not have data available prior to 1979.



Annual global mean surface temperatures with the effect of El Niño and La Niña (ENSO) events removed using the Foster and Rahmstorf (2011) approach, see methods at the end for details. Chart by Carbon Brief using Highcharts.
Removing the effects of El Niño from the temperature record makes 2017 rather than 2016 the warmest year on record for most temperature series, as 2016 temperatures benefited from a large El Niño event.

Temperatures for 2018 are bumped up modestly with the cooling effects of the early-2018 La Niña event removed, and 2018 would supplant 2015 as the third warmest year. Once El Niño effects are remove, the impact of major volcanic eruptions – such as those in 1982 and 1991 – are also much easier to identify in the temperature record.

https://www.carbonbrief.org/state-of...ld-warmed-2018