Hundreds of millions of people could be displaced by rising sea levels, crop yields could drop by up to one-fifth and communities could be devastated by floods, storms and heat waves, the report says. Many species of wildlife could face extinction as global temperatures are driven up by emissions of carbon dioxide and other greenhouse gases.
The report, written by more than 70 experts from 30 countries, is due to be published tomorrow (Monday) by the UN's Intergovernmental Panel on Climate Change (IPCC) following final negotiations at a meeting in Yokohama, Japan. Drawing on thousands of scientific studies, it is the most authoritative account to date of the manifold dangers of man-made climate change.
The Sunday Herald has seen a 40-page internal draft of the Summary for Policymakers, dated March 25. Though still under discussion, it gives a clear indication of what the IPCC is planning to say.
The report highlights eight major worldwide risks, including deaths, injuries, illnesses and disrupted livelihoods from floods, extreme weather events, food scarcity and water shortages. Livelihoods that depend on fishing and on endangered species could also suffer, it says.
"Throughout the 21st century, climate-change impacts will slow down economic growth, make poverty reduction more difficult, further erode food security, and prolong existing and create new poverty traps," the draft report says.
It warns that some changes will release more carbon, which will accelerate global warming. Medium and high-emissions scenarios "pose high risk of abrupt and irreversible regional-scale change in the composition, structure, and function of terrestrial and freshwater ecosystems," it says. This could happen in the Arctic tundra and in the Amazon rainforest, the report suggests. "These regional-scale changes would lead to substantial additional climate change."
Forests, a major store of carbon, are in danger across the globe, the report says. "Increased tree mortality and associated forest dieback will occur in many regions over the 21st century, due to increased temperatures and drought."
Global temperatures rises of four degrees above pre-industrial levels would "increase the likelihood of severe, pervasive, and challenging impacts", the report says. These include "the potential for adverse impacts on agricultural production and water resources worldwide, for potentially extensive global-scale ecosystem impacts, and for increasing substantial species extinction risk".
The alterations made in the leaked draft show that scientists are anxious not to be caught exaggerating, of which they have been accused in the past. They are also keen to stress the many actions that can be taken to cut climate pollution, known in IPCC jargon as mitigation.
"Substantial and sustained mitigation of greenhouse gas emissions over the next few decades can substantially reduce risks of anthropogenic climate change in the second half of the 21st century," the report says. But it adds: "Under all assessed scenarios for mitigation and adaptation, some risk of adverse impacts is unavoidable."
The IPCC report points out that climate change is already having a major effect on the world: "In recent decades, changes in climate have caused impacts on natural and human systems on all continents and across the oceans."
Patterns of rain and snowfall are changing, glaciers are shrinking and permafrost is thawing. Many species have shifted their geographical ranges. "Impacts from recent climate-related extremes, such as heatwaves, droughts, floods, cyclones, and wildfires, reveal significant vulnerability and exposure of some ecosystems and many human systems to current climate variability," the report says. There are early warning signs, it adds, that warm-water coral reef and Arctic ecosystems are already experiencing "irreversible regime shifts".
Perhaps the most unsettling section of the leaked IPCC report covers the risks to what it calls "human security". This is the first time the UN's experts have fully spelled out the dangers they foresee of conflicts, mass migrations and key infrastructure breakdown.
"Climate change can indirectly increase risks of violent conflicts in the form of civil war, and inter-group violence, by amplifying well-established drivers of these conflicts such as poverty and economic shocks," the draft says.
"The impacts of climate change on the critical infrastructure and territorial integrity of many states will influence national security policies." It further argues that rivalries between countries that lose land to the sea, or suffer water or food shortages, are liable to increase.
Coastal communities will become increasingly vulnerable to flooding and erosion. "By 2100, without adaptation, due to climate change and development patterns, hundreds of millions of people will be affected by coastal flooding with east, southeast, and south Asia particularly affected," the report says.
Some low-lying developing countries and small island states could face serious economic damage. "Climate change over the 21st century will increase displacement of people," it says. There are risks that extreme weather events may trigger the breakdown of infrastructure networks such as transport, communications and hospitals.
The IPCC report warns that rising global temperatures will harm major crops like wheat, rice and maize in tropical and temperate regions. It estimates that over the rest of the century overall yields could be reduced by up to 20%. "Climate change will progressively increase inter-annual variability of crop yields in many regions," it adds. "These projected impacts will occur in the context of rapidly rising crop demand."
As well as falling yields, climate change could damage other parts of food production and distribution systems. "Local temperature increases of 4°C or more above late 20th-century levels, combined with increasing food demand, would pose large risks to food security globally and regionally," it says. "Risks are greatest for tropical regions."
One of the risks pinpointed for urban populations is "water scarcity", especially for the less well-off. Rural communities could also suffer food shortages and income losses, as crops shift growing areas with the changing climate.
Valuable fisheries could disappear from some areas as stocks move due to rising water temperatures, the report warns. "Fisheries' catch potential will, on average, increase at mid and high latitudes, and decrease at tropical latitude."
The report predicts that, up until halfway through this century, the impact on human health will mainly be to exacerbate existant problems. "Throughout the 21st century, climate change will lead to increases in ill-health in many regions and especially in low and middle-income countries," it says. There will be a "greater likelihood of injury, disease, and death due to more intense heat waves and fires".
People are more likely to be malnourished in poor regions because of diminished food production. There are also "risks from lost work capacity and reduced labour productivity in vulnerable populations".
The report flags up the danger that diseases like malaria could spread as areas warm up. There will be "increased risks from food and water-borne diseases and vector-borne diseases," it says.
The outlook for wildlife is bleak, according to the report. "A large fraction of both terrestrial and freshwater species faces increased extinction risk under projected climate change during and beyond the 21st century," it says.
The changing climate will interact with other factors that are stressing animals and plants, like over-exploitation, pollution and invasive species. The risk of extinctions is increased under all emissions scenarios, the report concludes.
Shifting marine species "will cause high-latitude invasions and high local extinction rates in the tropics and semi-enclosed seas", it says. Rising temperatures will deprive some parts of the oceans of oxygen, creating "dead zones" for wildlife.
In medium and high-emissions scenarios, there are "substantial risks" to marine ecosystems from ocean acidification, the report warns. Coral reefs, and the plethora of species associated with them, are particularly vulnerable, as are polar regions.
"Climate change is the greatest threat to the planet's ecosystems and our lifestyles."
James Curran, chief executive of the Scottish Environment Protection Agency
"This report is telling us that if we don't accelerate our efforts to build a low-carbon economy and particularly a carbon-neutral energy system, we risk experiencing not just dangerous but unmanageable climate change.
"No major economy, including the UK, has yet begun to do that seriously."
John Ashton, a senior climate adviser at the Foreign Office in London from 2006-12, who was speaking to a conference in Glasgow organised by the Church of Scotland yesterday
"Climate change is one of the most serious challenges to global society and the natural environment both at home and abroad. If the world fails to act decisively, then the economic and social costs will be severe."
Paul Wheelhouse, the Scottish Government's Environment Minister, stressed the efforts being made in Scotland to cut climate pollution
"The Scottish Government is in denial on climate change. SNP ministers are unwilling to see the connection between their love of fossil fuels and their failure to meet climate targets year after year."
Green MSP Patrick Harvie
"The world has to give up fossil fuels as quickly as possible. Business as usual means a very grim future, especially if you live in a poor country."
Dr Richard Dixon, director of Friends of the Earth Scotland
"The impacts of climate change are not far away in the future. They are here and happening now, affecting the people, places and species we care about across every continent and ocean."
Gina Hanrahan, of WWF Scotland
"There are reports that one author had pulled out [of this study] because drafts were "too alarmist". Global warming is a long-term problem we must address. But describing it in one-sided, vaguely apocalyptic terms does not help us find smart solutions."
Dr Bjorn Lomborg, director of the Copenhagen Consensus Centre and a leading climate critic