WITHIN days the world’s leaders will flock to Paris for the latest round of UN climate talks. None will deny the consequences of the greenhouse effect on the Earth’s temperature, the role carbon dioxide plays therein or humanity’s part in adding to the level of that gas in the atmosphere over the past few centuries. The charts below help represent the state of climate change ahead of the negotiations.
The world is already 0.75°C warmer than it was before the Industrial Revolution. Recent studies, one published inScience and another in Climatic Change, suggest that a much-debated hiatus in global warming between 1998 and 2012 in fact never happened. The first says the cooler readings were caused by a switch to measuring ocean temperatures from buoys rather than ships; the second finds that the statistical tools used to demonstrate the apparent slowdown were inadequate. Meanwhile, the atmospheric concentration of carbon dioxide has risen from just under 340 parts per million (ppm) in 1980 to 400ppm today. Many see the effects of climate change in the fact that 2015 will likely be the hottest year around the world since records began—the current strong Niño, a Pacific-wide climatic phenomenon, is helping too.
The Arctic is heating up twice as quickly as the rest of the planet, and is the region where the impact of a warming climate can most vividly be seen. While a 1°C rise in temperatures at the equator will have some noticeable effects, in the Arctic such an increase melts the ice. Summer sea-ice cover has declined more than 40% over the past 36 years, causing huge changes both to local biology and to global meteorology. Since America bought Alaska from Russia in the 19th century, the average sea level there has risen by more than 20 cm and coastal villages in the state are threatened by the lapping of chilly waters. The sea is expected to rise much further by 2100. Warm periods in the past 3m years almost certainly saw increases of more than 5 metres according to the Intergovernmental Panel on Climate Change (IPCC). Were such a surge to happen again it would endanger cities such as New York, London and Mumbai.
Scientists have fretted about glacier melt for years, and over the West Antarctic Ice Sheet in particular since the 1970s. The Thwaites Glacier, the sheet’s “weak underbelly”, saw its grounding-line retreat 14 kilometres (8.7 miles) between 1992 and 2011, for example, and some fear nothing can be done to save it from collapse—irretrievable loss. Deep valleys in Greenland’s ice sheet may mean it melts much faster than once expected, hastening sea-level rise and disrupting ocean-circulation patterns. And shrinking glaciers in mountainous parts of the world, such as California and northern India, mean farmers lack meltwater for their crops. Studying observations over four decades, the IPCC is confident that glaciers, alongside the thermal expansion of warmer water in the ocean, explain 75% of the rise in sea level in the past century.