Image copyright AFP Image caption Changes in catch potential are likely to impact on fishing communities

A UN panel has released the most comprehensive assessment yet of the effects of climate change on our planet. Members of the Intergovernmental Panel on Climate Change (IPCC) say the "summary for policymakers" provides overwhelming evidence of the scale of these impacts. The BBC News website's science editor Paul Rincon breaks down the key findings.

"In recent decades, changes in climate have caused impacts on natural and human systems on all continents and across the oceans."

Most of the observed changes resulting from climate change concern natural systems, such as water resources, sea levels or biodiversity. But changes to human systems such as food production and livelihoods have also been attributed to global warming.

The report has medium confidence that global warming is altering patterns of rainfall or melting snow and ice, and that this is affecting water resources in terms of both quality and quantity.

The scientists also have high confidence that wildlife on land, in the rivers and in the oceans have shifted their geographic ranges, migration patterns and seasonal behaviour. Few extinctions have been tied to man-made global warming so far, but the report points out that much slower episodes of climate change over millions of years caused significant ecosystem changes and the disappearance of numerous species.

Previous studies have shown a mixture of impacts on food production, with some areas benefiting from changes, while others are hit hard. The authors say with high confidence that the negative impacts of climate change on crop yields have been more common than positive ones.

"Adaptation is becoming embedded in some planning processes, with more limited implementation of response."

Throughout history, people have tried to cope with a changing climate - with varying degrees of success, the report says. The document says that governments at various levels are beginning to develop adaptation plans and integrate climate considerations into existing programmes such as disaster risk management and water management.

What is the IPCC? In its own words, the IPCC is there "to provide the world with a clear scientific view on the current state of knowledge in climate change and its potential environmental and socio-economic impacts". The offspring of two UN bodies, the World Meteorological Organization and the United Nations Environment Programme, it has issued four heavyweight assessment reports to date on the state of the climate. These are commissioned by the governments of 195 countries, essentially the entire world. These reports are critical in informing the climate policies adopted by these governments. The IPCC itself is a small organisation, run from Geneva with a full time staff of 12. All the scientists who are involved with it do so on a voluntary basis.

The authors then present summaries of the steps different regions are taking to adapt to climate change. Progress has been variable. In Africa, for example, governments have started to build systems for disaster risk management and are taking basic public health measures. But it points out that such efforts have tended to be isolated so far.

In Europe, adaptation policies have been developed across all levels of government, while in North America, "incremental" planning is taking place, with some proactive adaptation to protect long-term investments in energy infrastructure. Oceania's adaptation efforts have been focused around sea level rise, and in southern Australia, on water scarcity.

The report stresses the importance of adaptation, explaining that the ways in which societies respond will influence the risks of climate change throughout the 21st Century. But the report warns that the effectiveness of adaptation can be limited. Continuing uncertainty about the severity and timing of climate impacts, as well as the long timeframes involved, complicate the decision-making process.

The report lays out a number of principles for effective adaptation. For example, planning and implementation can be enhanced through complementary actions by different governments.

Image copyright IPCC

"The overall risks of climate change impacts can be reduced by limiting the rate and magnitude of climate change."

In other words, we might not need to adapt so much if we tackle the causes of climate change itself. The risks associated with global warming are lessened under low temperature rise projections compared with those applying under the highest projections. However, under all the different temperature scenarios in the future, some risk from adverse impacts remains.

The document outlines the key risks of continued warming, according to criteria such as their magnitude, probability, or irreversibility. They have also been designed to comply with article 2 of the United Nations Framework Convention on Climate Change, which refers to "dangerous anthropogenic interference with the climate system".

The eight key risks refer to possible deaths, injuries and health impacts associated with, for example, flooding in low-lying coastal zones or heat waves. The breakdown of infrastructure following extreme weather events; the disruption of food production and supply; and the negative impact on communities that depend on threatened ecosystems - such as coastal fisheries - also make the list.

Furthermore, these risks are grouped into a smaller number of categories known as "reasons for concern". These include singular events, such as loss of the Greenland Ice Sheet, which would raise the sea level by a staggering seven metres.

"Climate change is projected to amplify existing climate-related risks and create new risks for natural and human systems."

Some of the risks associated with climate change will be limited to particular sectors or regions of the world, but some will have cascading effects.

The latter half of the report contains more detail on these impacts. The fraction of the global population experiencing water scarcity and the proportion affected by major river floods are expected to increase with the level of warming over the 21st Century. In addition, coastal and low-lying areas will experience submergence, flooding and erosion due to sea-level rise.

Land-based and freshwater wildlife species face an increased risk of extinction under the projected climate change scenarios for the century, especially as global warming interacts with other factors such as habitat changes, exploitation by humans, and invasive species. Under the high emissions scenarios, there is a "high risk of abrupt and irreversible regional-scale change" to those same ecosystems.

Fish and other marine animals will shift their ranges, causing invasions of high-latitude seas by low latitude species and local extinctions of marine animals in the tropics. This poses the risk of reduced supplies of fish with consequences for incomes and employment. Ocean acidification also poses substantial risks to marine ecosystems, particularly at the poles and at coral reefs.

For the major food crops, such as wheat, rice and maize, in tropical and temperate regions, global warming without adaptation is projected to negatively affect production for temperature rises of 2C or more above 20th Century levels. While positive effects are also expected, all aspects of food security are potentially impacted by climate change.

The report says the impacts on the global economy are difficult to estimate, but continued warming is expected to slow down economic growth, making efforts to tackle poverty more difficult. Human health and security - in the form of increased migration, increased conflict and implications for territorial integrity from, for example, inundation of land by rising seas - will also be affected by climate change.

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