Explainer: how scientists know climate change is happening

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We still control the future, however, as the magnitude of shifting weather patterns and the frequency of extreme climate events depends on how much more greenhouse gas we emit. We aren't facing the end of the world as envisaged by many environmentalists in the late 1980s and early 1990s, but if we do nothing to mitigate climate change then billions of people will suffer.

Causes of climate change

Greenhouse gases absorb and re-emit some of the heat radiation given off by the Earth's surface and warm the lower atmosphere. The most important greenhouse gas is water vapour, followed by carbon dioxide and methane, and without their warming presence in the atmosphere the Earth's average surface temperature would be approximately -20°C. While many of these gases occur naturally in the atmosphere, humans are responsible for increasing their concentration through burning fossil fuels, deforestation and other land use changes. Records of air bubbles in ancient Antarctic ice show us that carbon dioxide and other greenhouse gases are now at their highest concentrations for more than 800,000 years.

Evidence for climate change

The Intergovernmental Panel on Climate Change (IPCC) presents six main lines of evidence for climate change.

1. We have tracked the unprecedented recent increase in the amount of atmospheric carbon dioxide and other greenhouse gases since the beginning of the industrial revolution.
   
2. We know from laboratory and atmospheric measurements that such greenhouse gases do indeed absorb heat when they are present in the atmosphere.
   
3. We have tracked significant increase in global temperatures of at least 0.85°C and a sea level rise of 20cm over the past century.
   
4. We have analysed the effects of natural events such as sunspots and volcanic eruptions on the climate, and though these are essential to understand the pattern of temperature changes over the past 150 years, they cannot explain the overall warming trend.
   
5. We have observed significant changes in the Earth's climate system including reduced snowfall in the Northern Hemisphere, retreat of sea ice in the Arctic, retreating glaciers on all continents, and shrinking of the area covered by permafrost and the increasing depth of its active layer. All of which are consistent with a warming global climate.
   
6. We continually track global weather and have seen significant shifts in weather patterns and an increase in extreme events all around the world. Patterns of precipitation (rainfall and snowfall) have changed, with parts of North and South America, Europe and northern and central Asia becoming wetter, while the Sahel region of central Africa, southern Africa, the Mediterranean and southern Asia have become drier. Intense rainfall has become more frequent, along with major flooding. We're also seeing more heat waves. According to the US National Oceanic and Atmospheric Administration (NOAA) between 1880 and the beginning of 2014, the 19 warmest years on record have all occurred within the past 20 years; and 2015 is set to be the warmest year ever recorded.
   

What the future holds

The continued burning of fossil fuels will inevitably lead to further climate warming. The complexity of the climate system is such that the extent of this warming is difficult to predict, particularly as the largest unknown is how much greenhouse gas we keep emitting.

The IPCC has developed a range of emissions scenarios or Representative Concentration Pathways (RCPs) to examine the possible range of future climate change. Using scenarios ranging from business-as-usual to strong longer-term managed decline in emissions, the climate model projections suggest the global mean surface temperature could rise by between 2.8°C and 5.4°C by the end of the 21st century. Even if all the current country pledges submitted to the Paris conference are achieved we would still only just be at the bottom end of this range.

The sea level is projected to rise by between 52cm and 98cm by 2100, threatening coastal cities, low-lying deltas and small island nations. Snow cover and sea ice are projected to continue to reduce, and some models suggest that the Arctic could be ice-free in late summer by the latter part of the 21st century. Heat waves, droughts, extreme rain and flash flood risks are projected to increase, threatening ecosystems and human settlements, health and security. One major worry is that increased heat and humidity could make physical work outside impossible.

Changes in precipitation are also expected to vary from place to place. In the high-latitude regions (central and northern regions of Europe, Asia and North America) the year-round average precipitation is projected to increase, while in most sub-tropical land regions it is projected to decrease by as much as 20%, increasing the risk of drought.

In many other parts of the world, species and ecosystems may experience climatic conditions at the limits of their optimal or tolerable ranges or beyond. Human land use conversion for food, fuel, fibre and fodder, combined with targeted hunting and harvesting, has resulted in species extinctions some 100 to 1000 times higher than background rates. Climate change will only speed things up.

We don't have much time left

This is the challenge our world leaders face. To keep global temperature rise below the agreed 2°C, global carbon emission must peak in the next decade and from 2070 onward must be negative: we must start sucking out carbon dioxide from the atmosphere.

Despite 30 years of climate change negotiations there has been no deviation in greenhouse gas emissions from the business-as-usual pathway, so many feel keeping global warming to less than 2°C will prove impossible. Previous failures, most notably at Copenhagen in 2009, set back meaningful global cuts in emissions by at least a decade. Paris, however, offers a glimmer of hope.