Two degrees, at first glance, might seem an unremarkable, modest uptick in the temperature scale. But climate scientists warn that, in an increasingly warming world, this measure in fact represents a crucial tipping point.

The global average temperature has climbed 0.85 C (from 0.65 C to 1.06 C) during the period from 1880 to 2012, according to the UN’s Intergovernmental Panel on Climate Change. Climate scientists are warning that the industrialized world must now strive to hold the temperature to less than a two-degree increase by the end of the century.

It won't be easy. Britain’s Met Office, which monitors weather and climate change, warns the average global temperature will reach the 1 C increase from pre-industrial times this year for the first time, in part owing to the warming El Nino weather phenomenon.

In simple terms, this means the planet will be halfway to reaching the two-degree limit — which some scientists feel is certain to be breached.

Glaciers melt, ocean levels rise, crop yields falter

Lagoons that were once filled with rocky glaciers have over the course of decades become overgrown with lush grasses and shrubs, as seen in the images above of Alaska’s Pedersen Glacier. The glacier has pulled back by a considerable two kilometres, according to the U.S. Geological Survey. Isolated patches of snow can now only be found in this same area at much higher elevations.

While some contraction is expected, climate scientists say the warming ocean has increased the rate of melting. As a result, sea levels rise while coastlines sink.

The UN also warns that for every increase of one degree globally, grain yields fall by about five per cent — a considerable amount given the ever-growing global population. Advances in technology and management have meant that total yields of maize, wheat and other major crops have increased, but they would have increased more — by 40 megatonnes per year between 1981 and 2002 — in the absence of climate change.

"Each of the last three decades has been successively warmer at the Earth's surface than any preceding decade since 1850," warned a 2014 IPCC report which was compiled by hundreds of scientists around the globe.

"The period from 1983 to 2012 was likely the warmest 30-year period of the last 1,400 years in the Northern Hemisphere, where such assessment is possible."

Negotiating the target

The benchmark was first raised in the mid-1970s when Yale economist William Nordhaus remarked that a global temperature increase of more than 2 C would be unprecedented in human experience.

Climate change scientists have since applied the two-degree target more specifically but agreeing to commit to it has proved politically difficult. Governments debated the matter for decades before finally agreeing to the goal at the 2010 climate change conference in Cancun, Mexico.

At the same time, some countries have argued that the limit should in fact be a stricter target of 1.5 C. Other critics suggest limiting greenhouse gas emissions is a more effective means of controlling climate change.

The IPCC report set out a number of different scenarios, imagining a world at the end of the century that is 2 C and 4 C warmer.

It also includes a series of adaptation measures with suggestions for how each region might mitigate the risks. Substantial cuts in greenhouse gas emissions, which contribute to global warming, coupled with these mitigation measures are seen as the key to maintaining the 2 C goal.

By the end of the century, the panel says, CO2 and greenhouse gas emissions must register near zero — a mighty feat that some observers say is simply not achievable.