No one knows how much warming is "safe". What we do know is that climate change is already harming people and ecosystems. Its reality can be seen in melting glaciers, disintegrating polar ice, thawing permafrost, changing monsoon patterns, rising sea levels, changing ecosystems and fatal heat waves.

Scientists are not the only ones talking about these changes. From the apple growers in Himachal to the farmers in Vidharbha and those living in disappearing islands in the Sunderbans are already struggling with the impacts of climate change.

But this is just the beginning. We need to act to avoid catastrophic climate change. While not all regional effects are known yet, here are some likely future effects if we allow current trends to continue.

Relatively likely and early effects of small to moderate warming:

• Rise in sea level due to melting glaciers and the thermal expansion of the oceans as global temperature increases.

• Massive release of greenhouse gases from melting permafrost and dying forests.

• A high risk of more extreme weather events such as heat waves, droughts and floods. The global incidence of drought has already doubled over the past 30 years.

• Severe regional impacts. Example: In Europe river flooding will increase and in coastal areas the risk of flooding, erosion and wetland loss will increase substantially.

• Natural systems, including glaciers, coral reefs, mangroves, Arctic ecosystems, alpine ecosystems, Boreal forests, tropical forests, prairie wetlands and native grasslands, will be severely threatened.

•  The existing risks of species extinction and biodiversity loss will increase.

• The greatest impacts will be on the poorer countries least able to protect themselves from rising sea levels. There will be spread of disease and declines in agricultural production in the developing countries of Africa, Asia and the Pacific.

•  At all scales of climate change, developing countries will suffer the most.

Longer term catastrophic effects if warming continues:

• Greenland and Antarctic ice sheets are melting. Unless checked, warming from emissions may trigger the irreversible meltdown of the Greenland ice sheet in the coming decades, which would add up to a seven meters rise in sea-level over some centuries. New evidence showing the rate of ice discharge from parts of the Antarctic means that it is also facing a risk of meltdown.

• The slowing, shifting or shutting down of the Atlantic Gulf stream current is having dramatic effects in Europe, disrupting the global ocean circulation system.

• Catastrophic releases of methane from the oceans are leading to rapid increases in methane in the atmosphere and the consequent warming.

Never before has humanity been forced to grapple with such an immense environmental crisis. If we do not take urgent and immediate action to stop global warming, the damage could become irreversible.