Climate change is already being felt in all corners of the globe and some parts of the natural world may already be undergoing irreversible change, a major assessment by the UN Intergovernmental Panel on Climate Change has found.
The report on the impact of climate change – the first of its kind in seven years – stresses that the likelihood climate change will cause severe and irreversible damage to the planet grows if greenhouse gas emissions continue is high and the planet warms significantly.
The report is the result of years of work by a team of 309 lead global researchers. It is the second part of the IPCC's fifth assessment of climate change and focuses on its impact and how the world might adapt.
A 48-page summary released on Monday says some threats from climate change are considerable at just one or two degrees warming above pre-industrial levels. The average temperature across the globe has risen 0.85 since 1880.
The threat becomes high to very high under four degrees warming. Risks under this scenario include severe and widespread damage to unique and threatened human and eco-systems, substantial species extinction and threats to global food security.
The report identifies the world's poor as the most likely to be most under stress as climate hazards multiply the pressures already faced. But the impact of current extreme weather events, such as bushfires and floods, show a lack of preparedness in all countries, regardless of the level of development.
Dr Chris Field, co-chair of the research team behind the report, said climate change was not something that would happen in the future.
"We look around the world and see widespread impacts of the climate changes that have already occurred. Many of these have real consequences," he said.
"Vulnerability, the susceptibility to be harmed by climate change, is really widespread in society... there are vulnerable people, vulnerable activities, distributed around the world."
The report says there is still uncertainty about the timing and severity of the impact of climate change. It pitches the challenge as needing to identify the risks and deciding how to manage them.
The summary identifies eight global risks that it considers high probability and irreversible.
They include death, injury and disrupted livelihoods due to storm surges, coastal flooding and sea-level rise in low-lying communities; the breakdown of critical service such as electricity, water supply and emergency services due to extreme weather; food insecurity due to warming, drought flooding and extreme rainfall, particularly in poorer countries.
The report finds Artic-sea ice and coral reefs are at very high risk under two degrees of warming beyond that observed in 1986 and 2005.
The risk of extreme weather events such as heatwaves, extreme rainfall and coastal flooding is already considered moderate and will become high if temperatures rise another degree above 1986 to 2005 levels.
The report notes governments and industry are already beginning to change their practices to help key sectors adapt to a changing climate. In Australasia, planning for sea-level rise and reduced water availability is common. But while sea-level rise planning has evolved considerably over the past 20 years, its implementation remains piecemeal.
It says there are a wide range of measures that can help the world adapt to climate change and would result in a more robust, resilient and secure planet.
The IPCC had significantly more scientific research at hand than for its previous assessment in 2007. The report says impact due to climate change already observed includes:
• Changes to rainfall patterns and melting snow that are altering the quantity and quality of water systems.
• Land and sea species shifting their geographical range, seasonal activities, migration patterns and interaction.
• More negative than positive impact on crops yields.
• A relatively small burden on human health compared to other stresses.
• Significant vulnerability to extreme climate events, such as bushfires and heatwaves for some ecosystems and many human systems.
The report also assesses research into the projected future impact of climate change. Findings are that:
• Land and freshwater species face rising risk of extinction during and beyond the 21st century.
• The fraction of people facing water scarcity and affected by river flooding will increase in the 21st century as warming rises.
• Major food crops – rice, wheat and maize – are projected to be affected under temperature increases of two degrees above the levels of late-last century – though some individual locations may benefit.
• Global economic losses are difficult to estimate, but an additional two degrees warming could lead to between 0.2 to 2 per cent loss of global income. Losses are considered more likely to be greater than this range than smaller.
• Ill-health is expected to increase in many regions, especially developing countries.
The report follows the release last year of the first report looking at the physical science of climate change. A third part looking at options to cut greenhouse gas emissions will be released in mid-April.