The planet's temperature is rising, and it's rising at an accelerated pace that most scientists say is due to man-made factors. As the climate changes, they predict it will lead to more dramatic weather and disasters - not just droughts, but storms, floods and spreading disease too - and large numbers of people could be forced to find new homes as their environments are submerged, or food and water become scarce.

There's intense debate and a wide range of competing ideas on how to tackle what could be the most important issue of our time.

Global temperatures have risen by about 0.75 degrees Celsius in the past 100 years, and the U.N. Intergovernmental Panel on Climate Change (IPCC) says they are probably going to increase by between 1.8 and 4 degrees (3.2-7.2F) before the end of the century.

As the world heats up and glaciers melt, oceans are rising too. The U.N. climate panel predicted in a February 2007 report that sea levels would rise between 18 and 59 cm (7.2 to 23.6 inches) this century as a result of global warming. But a team of scientists warned in November 2009 that this estimate was too low, and the global sea level could increase by up to 2 metres by 2100 because as the pace of global warming accelerates more quickly than anticipated.

There isn't complete agreement on why the planet is getting warmer, but after examining all the evidence, the IPCC said there was at least a 90 percent probability it was primarily due to man-made factors.

"Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic (human) greenhouse gas concentrations," its report said.

Rising emissions of carbon dioxide and other greenhouse gases into the atmosphere trap heat, leading to global warming.

The IPCC says there's a greater than 90 percent chance there will be more heatwaves and "heavy rain events" this century, and a greater than 66 percent chance that more areas will be hit by drought and tropical cyclones will be more intense.

From a humanitarian perspective, the main issue is how projected changes in climate - and, by extension, weather events - could affect populations already made vulnerable by poverty and conflict.

A May 2009 report from the Geneva-based Global Humanitarian Forum (GHF) calculated that climate change already kills about 315,000 people a year through hunger, sickness and weather disasters, and the annual death toll is expected to rise to half a million by 2030.

The study said climate change seriously affects 325 million people every year, a number that will more than double in 20 years to an estimated 660 million.

Economic losses due to global warming amount to over $125 billion annually - more than the flow of aid from rich to poor nations - and are expected to rise to $340 billion each year by 2030, according to the report.

"Climate change is the greatest emerging humanitarian challenge of our time, causing suffering to hundreds of millions of people worldwide," said Kofi Annan, former U.N. secretary-general and president of the forum.

"The first hit and worst affected are the world's poorest groups, and yet they have done least to cause the problem."

The report said developing countries bear more than nine-tenths of the human and economic burden of climate change, while the 50 poorest countries contribute less than 1 percent of carbon emissions.

There's been a steep rise in hydro-meteorological disasters - which include floods, wave surges, storms, droughts and landslides - since the middle of the 20th century, according to statistics from the International Disaster Database managed by the Centre for Research on the Epidemiology of Disasters (CRED) at Belgium's Louvain University.

Because the pattern of weather-related disasters varies sharply from year to year, it is best to look at longer-term trends.

In the period from 1900 to 1909, 28 disasters were recorded in this category. Between 1950 and 1959, there were 232. And in the 1990s, there were 2,034. During the first five years of this century alone, there were 2,135.

However, CRED researchers caution that the increase in natural disasters isn't all down to global warming. "Climate change is probably an actor in this increase but not the major one," argues their report on statistics for 2007.

Another reason for the rise is the fact that information about natural disasters is more accurate than it used to be, according to CRED.

According to the 2008 Red Cross World Disasters Report, over the decade from 1998 to 2007, hydro-meteorological disasters accounted for 98 percent of the 2.82 billion people affected by disasters around the world, with floods affecting 45 percent and droughts 39 percent.

"Climate change is leading to an increase in the frequency and intensity of extreme weather events, like cyclones, droughts and floods across much of our globe," said Madeleen Helmer, head of the Red Cross Red Crescent Climate Centre.

Aid agency Oxfam said in a 2007 report that weather-related disasters have quadrupled over the last two decades. It pointed to a rise in small- and medium-scale disasters as a particularly worrying trend.

And the charity warned in an April 2009 report that the world's relief agencies will be overwhelmed by a sharp rise in the number of people affected by climate-related disasters by 2015 unless the quantity and quality of aid improves.

In 2007, the U.N. Office for the Coordination of Humanitarian Affairs (OCHA) issued an unprecedented 14 funding appeals for disasters caused by extreme weather.

The IPCC says the areas of the world most vulnerable to climate change are Africa, Asia's mega-deltas and small island states like Tuvalu, in the south Pacific.

Aid agencies are concerned that a rise in climate-related disasters will tip a growing number of poor people who live on the edge of survival into crisis.

Humanitarians fear that short- and long-term environmental damage could lead to food and water shortages. These, in turn, may create or exacerbate tensions between communities, which could spill over into violent conflict.

The threat climate change poses to world security garnered growing attention in 2007. U.N. Secretary-General Ban Ki-moon warned that environmental changes caused by global warming "are likely to become a major driver of war and conflict", and climate change was debated at the Security Council for the first time.

The 2007 Nobel Peace Prize was awarded jointly to the IPCC and former U.S. Vice President Al Gore "for their efforts to build up and disseminate greater knowledge about man-made climate change".

Yet some experts who've spent time studying the causes and drivers of wars argue that climate change and its associated effects alone probably won't spark conflict, but could add to tensions in already unstable areas.

Flooding, sea level rises and land degradation could also force people from their homes, creating tens of millions of climate change migrants.

There is no single authoritative prediction about the numbers of people who could lose their homes because of climate change.

L. Craig Johnstone, the U.N. Deputy High Commissioner for Refugees, warned in December 2008 that the impacts of climate change could uproot around six million people each year, half of them because of weather disasters like floods and storms.

He added that aid agencies would need to boost the relief supplies they keep in stock for emergencies by 10 to 20 times.

On a longer view, British-based aid and development agency Christian Aid says we could be talking about 250 million people displaced by 2050.

That's based on an updated estimate from scientist Norman Myers, who suggested in 1995 that between 150 and 200 million people would have to permanently leave their homes because of climate change. Myers told Christian Aid in March 2007 he believed the correct figure would be closer to 250 million.

Oli Brown, a programme manager with the International Institute for Sustainable Development, wrote in an October 2008 article for Forced Migration Review that such estimates need more work.

He argues that getting a better handle on climate-related migration will require an effort to develop "objective and empirically-based detailed numerical scenarios". And to produce these, we need more advanced computer models and better data on migrants' movements within and across national boundaries.

Most experts agree that much of the displacement caused by climate change is likely to be within countries. For those who do cross national borders, it's still unclear who should help them.

Climate refugees don't have the same protection under international law as those who flee their countries to escape violence.

AlertNet has a tipsheet explaining the key issues.

Scientists expect spreading deserts and degradation of farm land related to climate change to pose a serious threat to food supplies for the world's surging population in coming years.

Africa, Latin America and parts of Asia - where the climate is already more extreme and arid regions are common - are likely to be most affected as rainfall declines and its timing becomes less predictable, making water more scarce.

In some regions the expansion of deserts and salination of once arable land is already well under way. In the future, this is likely to be most widespread in drier areas of Latin America, including in farming giant Brazil.

In Africa, increasing climate variability is expected to create major problems for already poor farmers in rural areas, who are likely to see their growing seasons get shorter and crop yields cut, especially in areas near already arid and semi-arid regions.

The IPCC says that between 75 million and 250 million Africans could find their water sources at risk from climate change by 2020. Agricultural production could fall significantly, and in some countries the yields from rain-fed agriculture could be reduced by up to 50 percent, according to the U.N. panel.

Experts emphasise that climate risks are so high in Africa not just because the continent is likely to experience more extreme weather events, but also because of the additional stresses and challenges it faces, like conflicts and corruption.

In Asia, global warming may also wipe out large areas of glaciers in the Himalayas and surrounding highlands, threatening livelihoods across much of the continent. Rising temperatures have already shrunk glaciers on the mountains dividing China and South Asia.

More rapid melting could severely disrupt river flows, rainfall patterns and farming across Asia, because glaciers across the Himalayas and the Qinghai-Tibet Plateau are a major source for rivers such as the Yangtze in China, the Mekong in Indochina and the Ganges in India. Glacier-fed rivers could swell as the ice melts, but then dry out as the ice disappears.

Global warming is likely to lead to shifting patterns of disease, bringing new health hazards for millions.

According to the May 2009 Human Impact Report from the Global Humanitarian Forum, every year the health of 235 million people is likely to be seriously affected by gradual environmental degradation due to climate change.

The report said World Health Organisation (WHO) models suggest global warming will account for 150,000 deaths annually from malnutrition, 95,000 from diarrhoea and 55,000 from malaria.

In 2008, the WHO warned that climate change would exacerbate health crises in many poor countries already strained by inadequate hospitals, too few medical staff and uneven access to drugs.

AlertNet has a tipsheet summarising the expected impacts of climate change on human health. These include:

• Wetter, warmer weather could take malaria into previously cool areas like the highlands of Tanzania and Rwanda, as well as boosting malaria-carrying mosquito populations in Africa's tropical zones.
• Increased rainfall in hot areas could boost dengue fever and Rift Valley Fever - also carried by mosquitoes - and leishmaniasis, a parasitic disease that causes blindness.
• Hot, fetid conditions encourage cholera - a waterborne disease that flourishes when floods contaminate drinking supplies with sewage - as well as diarrhoea and other stomach infections.
• Meningitis, passed through the air by coughing, sneezing and breathing, could increase in hot dusty regions such as the Sahel if climate change brings more arid conditions.
• More frequent and intense heatwaves may lead to an increase in illness and death, particularly among young, elderly and frail people, and especially in large urban centres. This occurred during the European heatwave of 2003, which is estimated to have killed at least 35,000 people.
• Increased smog and air pollution could cause more respiratory disorders.

The WHO says confronting the health challenges from global warming will require increased efforts to forecast changing weather patterns, fight mosquitoes and other disease-spreading bugs, distribute vaccinations and boost medical coverage.

Deforestation contributes around 20 percent of total greenhouse gas emissions - more than all the world's cars, trucks, trains and airplanes put together.

Environmental groups claim that protecting tropical forests from cutting and burning is the most direct and fastest way to mitigate some of the impact of climate change - a practice known as "avoided deforestation".

Increased logging in the Amazon, and parts of Asia and Africa, has raised concern - not just because it reduces the world's capacity to store carbon. Trees act as a natural defence against soil erosion, flooding and landslides, and cutting them down can increase the risk of disasters.

There is also concern that logging is planned and carried out with little respect for the rights of indigenous and other minority groups that rely on forests for their livelihoods and survival.

Countries grouped in the Forest Eight (F8) - Brazil, Cameroon, Congo, Costa Rica, Gabon, Indonesia, Malaysia and Papua New Guinea - argue they should be compensated for the economic losses they would incur by limiting their logging.

At international level, discussions continue on how best to achieve "reduced emissions from deforestation and degradation" (REDD). Its proponents want to make emissions cuts from forest areas eligible for global carbon trading.

The World Bank has launched a financing mechanism called the Forest Carbon Partnership Facility which will pay developing countries to protect and replant tropical forests.

By creating economic value for forests, it aims to help developing countries generate new revenue for poverty alleviation while maintaining the forests' natural benefits such as fresh water, food and medicines.

Indonesia is already getting some international financial help to preserve and restore peatlands that were drained in a scheme to create rice fields. The dense tropical swamps release large amounts of carbon dioxide when burned or drained to plant crops, including palm oil for biofuels.

For the first time in history, half the world's 6.7 billion people live in urban areas, according to the United Nations.

Rapid urbanisation, combined with factors like climate change, is increasing the chances of a mega-disaster that would dwarf anything the world has seen before. Six of the world's 10 most populous cities are on or near the coast, leaving millions vulnerable to flooding, storm surges and post-quake tsunamis.

In 2007, U.N. emergencies chief John Holmes told an international conference on disaster prevention: "The combination of decaying infrastructure, land erosion, crowded conditions and a lack of rescue services could lead to a catastrophe on an unprecedented scale. Global warming makes this task still more urgent, and our responsibility ever more grave."

Mumbai in India, Mexico City, Colombia's capital Bogota, China's Shanghai, Manila in the Philippines and the Indonesian city of Jakarta are just some of the places of particular concern to disaster experts.

One-tenth of the global population - 634 million people - live in risky coastal areas that lie less than 10 metres above sea level, and the number is rising, according to a 2007 study by academics at the International Institute for Environment and Development (IIED) and New York's City and Columbia universities.

Three quarters of them are in Asia, with researchers singling out China and Bangladesh as particularly high risk. Twenty-one nations - including 16 small island states - have more than half their population in the zone. And nearly two-thirds of urban settlements with over 5 million inhabitants are partially in the zone.

"Migration away from the zone at risk will be necessary but costly and hard to implement, so coastal settlements will also need to be modified to protect residents," said IIED's Gordon McGranahan.

The U.N.'s State of the World's Cities 2008/2009 report noted that 3,351 of the world's cities are located less than 10 metres above sea level, putting nearly 400 million people at risk of displacement from rising sea levels.

As the World Meteorological Organisation points out in its book, "Climate into the 21st Century", while climate statistics normally focus on averages, people tend to be most interested in extreme weather events, including super-strength storms and long spells of abnormal weather like severe winters and hot summers.

Weather refers to what is happening to the atmosphere at a given time - usually on a daily or weekly basis. Climate is a measure of what to expect in any month, season or year, and is based on statistics built up over a longer period.

Scientists and weather experts often use the term "climate variability", which is different from "climate change". According to the International Research Institute for Climate and Society (IRI), climate variability refers to "variations of the climate system, which includes oceans and the land surface as well as the atmosphere, over months, years and decades" - not all of which can be predicted.

Climate change, in comparison, refers to "longer-term trends in average temperature or rainfall or in climate variability itself, and often to trends resulting wholly or in part from human activities, notably global warming due to the burning of fossil fuels".

Inevitably, there's a high degree of uncertainty in long-term predictions. Yet many climate and development specialists believe that, by learning how to manage climate variability from season to season and year to year, communities will be better equipped to adapt to climate change in the future. This requires well-designed policies, as well as access to high-quality data and information about climate.

In Africa - which is highly vulnerable to climate change because of its dependency on agriculture - farmers often lose crops because they can't get hold of accurate and timely weather and climate forecasts.

The IRI says the opportunities for incorporating climate information into development work are still largely being missed in sub-Saharan Africa. But its website highlights some examples of initiatives where climate risks are being tackled - both on an "everyday" basis, as with agriculture in Mali, and for "one-off" extreme events, like flooding in Mozambique.

International, national and local-level activities to tackle climate change fall into two main categories: mitigation and adaptation.

Mitigation is concerned with limiting greenhouse gas emissions and finding ways to reduce the impact of global warming - for example, through energy efficiency and switching to renewable energy sources.

Adaptation assesses the vulnerability of societies and natural systems to the consequences - negative and positive - of climate change, and aims to help people cope with those effects.

IPCC scientists said in 2007 that growth in greenhouse gas emissions could be curbed at reasonable cost to the world.

Greenhouse gas emissions have risen by 70 percent since 1970, and will rise by between 25 percent and 90 percent over the next 25 years if the world continues with "business as usual".

The IPCC says rich nations need to cut emissions by 25-40 percent below 1990 levels by 2020 to keep temperatures below what some nations see as a "dangerous" 2 Celsius (3.6 Fahrenheit) rise.

The U.N. panel says measures that could help reduce emissions include boosting renewable energy, reducing deforestation and improving energy efficiency.

In December 2007, at a major meeting in Bali, nearly 200 nations agreed to launch negotiations on a new pact to replace or extend the Kyoto Protocol, which binds 37 rich nations to cap emissions of greenhouse gases until 2012 but does not include the United States.

Washington said the Bali "roadmap" marked a new chapter in climate diplomacy after six years of disputes since President George W. Bush refused to sign the Kyoto Protocol, saying it would cost too much and wrongly omitted binding targets for developing nations.

The Bali document widens Kyoto to the United States and developing nations such as China and India. A successor pact is due to be agreed at a meeting in Copenhagen in December 2009, to allow three years for ratification by national parliaments before the expiry of the Kyoto Protocol.

Some of the world's fastest-growing economies, with the largest populations - like China and India - are increasingly responsible for carbon emissions. But they have been reluctant to make promises on limiting them without seeing more decisive steps from wealthier industrialised nations first.

After Democratic Party candidate Barack Obama was elected U.S. president in November 2008, he vowed to adopt an aggressive approach to global warming. That raised hopes around the world of reaching a new global deal in Copenhagen, although progress towards that goal has been slow.

At a G8 summit in Italy in July 2009, rich and poor nations acknowledged that global temperature rises should be limited to 2 degrees Celsius, and G8 nations set a new goal of cutting their overall emissions by 80 percent by 2050.

But the world's leading economic powers failed to persuade China and India and other developing nations to sign up for a global target of halving world emissions by 2050.

Developing nations first want rich countries to specify interim targets for 2020 on the way to the mid-century goal, and to agree on ways to raise tens of billions of dollars in new funds to help poor nations combat the effects of climate change.

Poorer countries and aid groups are calling for emissions cuts of 40 percent or more from 1990 levels by 2020.

The European Union has promised to reduce emissions by at least 20 percent by 2020 from 1990 levels, and U.S President Obama aims to cut U.S. emissions back to 1990 levels by the same deadline. Japan has announced a 2020 target equivalent to a cut of just 8 percent from 1990 levels.

Green groups say these interim pledges are not ambitious enough.

There is also concern that the financial turmoil that hit the world's markets in the autumn of 2008 could weaken wealthy countries' will to implement policies to curb their emissions.

In response to this, U.N. climate chief Yvo de Boer argues the global credit crisis could actually speed up countries' efforts to create "green growth" industries by revamping the financial system behind them. He says a successful outcome to climate change negotiations would create new environmental markets, investment opportunities and jobs.

While negotiations on emissions cuts are essential to limit global warming in the future, development and relief workers say vulnerable communities must be helped to adapt to the climate variability that's already happening.

Protecting people from the damaging consequences of climate change and weather disasters includes sophisticated measures like longer-term forecasting and switching to more appropriate crops. Or it can be as simple as giving people early warning of weather hazards, storing food in a safe place and building toilets away from flood-prone land.

A growing number of governments recognise the need to bolster their capacity to prepare for and manage climate-related disasters. From Pakistan to Mozambique, governments are setting up agencies to deal with disasters and reduce the risks to their populations.

The measures they can take include:

• Better analysis and use of data and forecasting to give the public early warning
• Physical barriers, such as better flood defences
• Financial buffers, such as cash transfers and weather-related insurance
• Supporting a more pro-active international relief and response system

Disaster experts are also devising ways to ensure that important weather and climate information filters down to those most affected, as well as analysing communities' own views of the hazards they face. Traditional knowledge is valuable in avoiding disasters, but it has often been overlooked by scientists and aid agencies.

The Red Cross has launched intensive consultation processes in villages in Nicaragua and Vietnam, for example, in which local people assess the dangers they face. The results are combined with scientific data on weather patterns and used as a basis to draw up measures to reduce the risk of disasters.

In recent years, most aid agencies have made progress in thinking about how to limit the potential impacts of climate change when planning humanitarian and development work. Some are identifying and reducing the risks to their projects and other assets - a process known in the jargon as "climate proofing".

It's now widely recognised that developing countries need substantial financial support to cope with climate change and develop in a cleaner way than industrialised nations. But the issue of who should pay for the technology, infrastructure and new institutions required remains controversial.

International funds and schemes, like the U.N.Clean Development Mechanism (CDM), have been established to channel money to renewable energy initiatives in developing countries.

The CDM gives carbon credits to investors in projects that cut greenhouse gas emissions in developing countries, such as wind farms and hydro-power projects. A 2 percent levy on CDM transactions goes into the U.N. Adaptation Fund, which was set up to help poorer nations adapt to global warming.

The fund is expected to increase from around $80 million in 2009 to around $300 million per year by 2012, although this amount depends on fluctuations in carbon prices.

Yet this sum is paltry compared with calls from developing countries, the United Nations and aid groups for tens of billions of dollars per year in new financing for climate change adaptation.

Campaigners say new ways to raise money will have to be found, including extending the adaptation levy to other mechanisms and international emissions trading.

The 2008 launch of two big World Bank climate investment funds - to help developing countries introduce low carbon technologies and adapt to climate change - sparked rows over who should control the money, and whether it should be provided as loans or grants.

Oxfam has argued that G8 countries owe around 80 percent of the $50 billion or more it estimates developing countries need each year to adapt to the harmful effects of climate change.

The aid agency says more innovative solutions may be needed to raise funds on the scale required, including shipping and aviation taxes, and larger levies on carbon trading.

Other market-based ideas for protecting countries, communities and individuals against climate and disaster risk range from catastrophe bonds to micro-insurance and disaster derivatives.

Both vulnerable countries and aid groups insist that funding to tackle climate change must be in addition to development aid, and poorer countries must be given a fair say in how the money is spent.

The December 2008 U.N. climate change talks in Poland ended on a sour note, with rich nations accused of meanness after they put off a decision on ways to raise more money for adaptation.

British Prime Minister Gordon Brown said in mid-2009 that rich countries should be prepared to stump up $100 billion per year by 2020 to help poor countries cope with the impacts of climate change and curb their emissions, although Oxfam says the annual sum required is more in the region of $150 billion.

The world's least-developed countries want immediate financing of up to $2 billion to pay for urgent adaptation measures they have already identified.

But with only several hundred million dollars having been made available so far, a trust gap has opened up between rich and poor countries over the lack of international funding to help vulnerable states.