But we now have a better idea of how bad 1.5 degrees C of warming looks, since we've already heated the world by about 1 degree Celsius over pre-industrial levels by sending greenhouse gases like carbon dioxide into the atmosphere.
A landmark United Nations report paints a dire picture of the catastrophic consequences the world will face if immediate action is not taken to limit the global warming to 1.5°C, warning that at 2°C, the world could see 10 cm more global sea level rise, loss of all coral-reefs and worsening food shortages. "If the current warming rate continues, the world would reach human-induced global warming of 1.5°C around 2040". Countries in the southern hemisphere will be among the worse off, the report said.
"One of the key messages that comes out very strongly from this report is that we are already seeing the consequences of 1°C of global warming through more extreme weather, rising sea levels and diminishing Arctic sea ice, among other changes", Co-Chair of one of the IPCC Working Groups Panmao Zhai said.
While more than 180 countries have accepted the report's summary, the U.S. (which is the second biggest emitter in the world) said that their acceptance of the report does not "imply endorsement" of the findings. For example, by 2100, global sea level rise would be 10 centimeters lower with global warming of 1.5 degrees Celsius compared with 2 degrees Celsius.
Under pressure from island nations at risk from sea-level rise, the United Nations agreed during the Paris negotiations to ask the IPCC to investigate the impact of 1.5 degrees of global warming.
Coral reefs would decline by 70 per cent to 90 per cent at 1.5 deg C, whereas virtually all would be lost at 2 deg C.
Countries around the world must now act to completely cut out global carbon emissions by 2050, the UN-backed research says, or risk mass food shortages, more floods, droughts, storms. The report also examines pathways available to limit warming to 1.5 °C, what it would take to achieve them and what the consequences could be.
The WWF called on the European Union to take urgent action to limit global warming to 1.5ºC, saying in a press release: "Approved by 195 governments, the report underscores the small window of opportunity we have to make immediate, deep and transformational changes - without which the world we know will be irreversibly changed".
The report calls for huge changes in land, energy, industry, buildings, transportation and cities. Global net human-caused emissions of carbon dioxide would need to fall by about 45 percent from 2010 levels by 2030, reaching "net zero" around 2050.
"E$3 ven with erroneous attribution of extreme weather/climate events and projections using climate models that are running too hot and not fit for objective of projecting 21st century climate change, the IPCC still has not made a strong case for this massive investment to prevent 1.5C warming", she said on her Climate Etc. blog.
Pompeo heads to North Korea for nuclear talks
That left Seoul lobbying hard for a second summit between Trump and Kim to keep alive a positive atmosphere for nuclear diplomacy. North Korea is clear to make sure that the United States understands that these feelings are very reciprocal.
If the average global temperature temporarily exceeded 1.5C, additional carbon removal techniques would be required to return warming to below 1.5C by 2100.
The review of thousands of scientific papers also said the spread of disease and economic damage and harm to yields of crops will be less severe at 1.5C than 2C, as will the extinction of species. "Every bit of extra warming makes a difference", said Abdalah Mokssit, director of Morocco's National Meteorological Department and IPCC secretary.
The report emphasises the need for placing climate change at the centre of all national and worldwide agendas. As many as 10 million fewer people would be exposed to risks like flooding.
Experts have said "unprecedented" should take place and said the report should urge governments to invest in clean growth and renewable energy.
Some impacts of climate change show particularly large jumps between 1.5 and 2.0°C.
The landmark Paris Agreement adopted in December 2015 by 195 nations at the 21st Conference of the Parties to the UN Framework Convention on Climate Change (UNFCCC) included the aim of strengthening the global response to the threat of climate change by "holding the increase in the global average temperature to well below 2°C above preindustrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels".
While warming of 2C above pre-industrial levels has widely been thought of as the threshold beyond which unsafe climate change will occur, vulnerable countries such as low-lying island states warn rises above 1.5C will threaten their survival.
The IPCC report was released just as this year's Nobel Prize for Economics was awarded to two professors studying the economic impact of climate change.
This report shows the longer we leave it to act, the more hard, the more expensive and the more unsafe it will be.