Annual State of the Climate Report for 2017

Last year (2017) was another warm year for the global climate. Although it did not break the global temperature record, it was still the second or third warmest in the instrumental record, warmer than any year before 2015. Notably, 2017 was also the warmest year where the global temperatures were not enhanced by an El Niño. Warmth in the climate system was seen across a range of indicators, from high in the atmosphere to the ocean waters, and across all parts of the globe. Furthermore, 2017 was another year when the levels of dominant greenhouse gases reached new peaks in the instrumental record.

All of these data are presented in the 'State of the Climate in 2017' report, the 28th consecutive instalment to the annual reports produced by the American Meteorological Society. These comprehensive reports are led by scientists from the NOAA National Centers for Environmental Information, with contributions from over 500 scientists from all over the world. Met Office scientist Robert Dunn leads the Global Climate chapter with Kate Willett as one of the three co-editors, with contributions from several other Met Office scientists.

The report provides a detailed update on global and regional Essential Climate Variables (ECVs) using a wide range of climate monitoring records, including several produced by the Met Office Hadley Centre and provided on the HadOBS data portal. Patterns, trends and changes are tracked across the global climate system. This year's report and all previous editions are freely available on the Americal Meteorological Society website.

Temperature

2017 was exceptionally warm at a global scale throughout the climate system.  Not only was the global average surface temperature the second or third warmest in the instrumental record, but temperatures were also unusually high in other parts of the climate system – the global lower tropospheric temperature, sea-surface temperatures, and global upper ocean heat content were all at record or near record high values.

Modes of Variability

During 2017, the El Niño Southern Oscillation (ENSO) exhibited neutral conditions for most of the year, turning into a weak La Niña in the last few months.  During 2015-16 a strong El Niño boosted global temperatures to make 2015 and then 2016 in turn the warmest years in the instrumental record.  In contrast 2017 had no enhancement to the global temperatures but was still the second or third warmest on record.  In the Northern Hemisphere, the North Atlantic Oscillation was, on the whole, in a positive phase, with a generally more westerly airflow, resulting in mild winters for Europe during 2016/17 and early 2017/18.

Snow and Ice – the Cryosphere

The continued higher than average temperatures have contributed to the ongoing decrease in glacier volume.  Preliminary results for 2017 indicate that it is the 38^th consecutive year that glaciers have lost more ice than they gained. The loss since 1980 is now the equivalent of taking a 22m slice off the top of the average glacier.  In the Arctic, the maximum sea-ice extent was at a record low, 8 percent below the 1981-2010 average. And in the Antarctic, a record low sea-ice extent was observed from January through to April.

Water

Every month of 2017 had at least 3% of global land area experiencing “severe” drought, the fourth greatest extent since 1950 (after 1984, 1985 and 2016). In British Columbia, drought conditions contributed to its most extensive wildfire season on record.  For extremely heavy precipitation, both Severe Tropical Cyclone Debbie in eastern Australia and Hurricane Harvey in the U.S. produced unprecedented impacts from heavy rainfall, with Harvey breaking records for 5-day rainfall accumulations by almost a factor of two for some locations in Texas.

Atmospheric Chemical Composition

The atmospheric concentrations of the key long-lived greenhouse gases (CO₂, CH₄ and N₂O) rose to new record levels during 2017. The global annual average CO₂ concentration was 405.5 parts per million (ppm), 2.2ppm above the 2016 value.  This is the highest in the modern record going back to the mid-20^th century, and also in ice core records dating back as far as 800,000 years. While greenhouse gas concentrations increased, emissions and abundance of most ozone-depleting substances (like CFCs) continued to decline.  The annual mean total stratospheric ozone levels in 2017 were above the 1998-2008 average for almost the entire Southern Hemisphere indicating a slight reduction in the size of the ozone hole.  Also, concentrations of fine particulates (aerosols) over some highly populated areas were lower in 2017 than in 2016.

Natural Environment

The comprehensive set of observations in the State of the Climate report demonstrates the interconnected nature of the Earth’s climate system.  This year’s Global Climate chapter included a sidebar on phenological observations, which measure the response of the natural environment to climate drivers via the dates of bud burst, lake water chlorophyll concentrations and plant ageing in autumn. Many phenological events provide clear indicators of the influence of climate on our environment and natural resources. Studies in Germany have showed that tree leaves are unfurling earlier in the year, and in Lake Windermere (UK), algal blooms are occurring earlier in spring. 2017 also saw differences in the start and end of the growing season across the Northern Hemisphere. North-eastern Europe and Russia experienced a 6 day delay in the start of the growing season associated with a Spring cold spell, whereas in North America the growing season arrived 5 days earlier than average.