2017: On course for one of the warmest years ever
2017 looks set to be one of the three or four warmest years on record. In fact, for the period up to the end of August, it was ranked the second warmest. Barring any dramatic changes therefore, all 17 years since 2001 will number among the 18 warmest years ever. Scientists are virtually unanimous that this is a clear signal of climate change.
Each month in 2017 up to August has been among the three warmest since measurements began in 1880, making this period, which featured a departure of 0.88°C from the 20th century average, the second warmest for these months after 2016 (1.01°C). The forecast corridor indicates that 2017 will ultimately turn out to be the third warmest year (or by a whisker the fourth) after 2016 and 2015. This is supported by the increased probability that the final months of 2017 will see a slightly cooling La Niña phase – a phenomenon of ENSO, the natural climate oscillation in the Pacific. Up to September, the year had been characterised by neutral ENSO conditions – and the forecasts from international models in the spring anticipating an El Niño phase in the second half of the year proved unfounded. El Niño conditions tend to lead to higher global temperatures.
Overall, the greatest positive temperature anomalies occurred between January and August in central, northern and eastern parts of Asia, where temperatures during the winter and spring were already much too warm. Similarly, average temperatures in eastern and southern regions of North America up to the month of August were also too high – mirroring the anomalies experienced in the winter and parts of the spring. A further land area exhibiting positive anomalies was western Europe, along with central and southern parts of the continent. In March in particular, it was excessively warm in these regions, then again during the periods of hot weather in June and (in southern Europe at least) August as well.
Western Russia was slightly cooler than the 20th century average temperature – the result of monthly negative anomalies in this region and neighbouring areas in the months of April, May, June and July. In April especially, western, central and eastern Europe, which up to then had experienced abnormally warm weather, came under the influence of a high-altitude trough filled with Arctic air, resulting in extensive frost damage in many parts of the continent just when the flowering phase in fruit and wine growing regions was already at a very advanced stage.
In the equatorial Pacific, the first few months of the year saw sea surface temperatures featuring tremendous heat off the coast of Peru and relative cool in western areas of the equatorial Pacific. In February, March and at the start of April, this led to severe and destructive rainfall in northwestern regions of Peru, favoured by the extreme humidity from the high rate of evaporation from the sea. The Peruvian weather service spoke of a “coastal El Niño” – an El Niño phase that has not fully developed. In addition, as a result of teleconnection effects, this equatorial temperature pattern contributed to an extremely active early thunderstorm season in the USA, which also caused substantial damage.
In Europe, the summer months saw an approximate north-south split in terms of rainfall: in southern Europe from Portugal and Spain to southern France, and especially in Italy and parts of the Balkans, the weather was excessively dry and generally too hot, contributing to numerous forest fires. In more northerly regions, on the other hand, it was predominantly too wet. For example, the upper Loire catchment area in France, and the northeast of Germany were afflicted by torrential rainfall, while Scotland experienced its wettest June since records began.
In China, as in the previous year, intense precipitation during the East Asian monsoon caused river flooding and damage in June and July. The South Asian monsoon, which for decades has been bringing increasingly heavy rainfall to places like central India, resulted in severe flooding in India, Nepal and Bangladesh until very late in the monsoon season in September. This offers a glimpse into the future of monsoon systems: as climate change progresses, the IPCC expects a more prolonged monsoon season, and a further increase in the variability and intensity of precipitation, as well as of 5-day rainfall amounts.