Climate change is the greatest challenge facing humanity. The world's leaders and citizens can and must face up to this fact and deal with it, and they have three distinct time frames in which to act.

The first period, spanning the next five to 10 decades, is the longest and is the time frame of our children and grandchildren. If greenhouse-gas emissions continue unabated over this period then global temperatures will rise by several degrees. This would have globally catastrophic impacts, including severe sea-level rise and more deadly heat waves, droughts, floods and more intense hurricanes.

We can limit these impacts, but only if we act now. We need to reduce and then reverse the concentration of greenhouse gases in the atmosphere back to safe levels. This will entail being less wasteful with resources, avoiding deforestation, being more efficient with energy, and using more renewable sources of power such as wind and solar. The major emitting countries, including the two biggest, the United States and China, as well as others, both developed and developing must act now if we are to avert catastrophe over the coming decades.

The second period – the time frame of this generation – is the next two decades, during which a global temperature increase of at least one degree Centigrade is already inevitable. Even if all global emissions were to miraculously stop tomorrow, enough greenhouse gases have already accumulated in the atmosphere to mean that some climate change is unavoidable.

The most vulnerable countries include the 50 Least Developed Countries, the small island developing states and most of Africa. These three groups consist of nearly one hundred countries (some are in more than one group) with a total population of nearly a billion people. Their combined greenhouses-gas emissions are less than 3% of the global total, but they will certainly suffer the most adverse impacts of climate change.

Of course they will not be the only ones to suffer. There are also significant vulnerable communities in wealthier developing countries such as China and India and in even the richest countries, as the poor communities of New Orleans who experienced Hurricane Katrina in August 2006 can testify.

Vulnerable countries and communities must prepare for climate change impacts through adaptation and they will need help. Richer countries must take responsibility for having caused the problem. They can help the more vulnerable, primarily through funding but also through other means such as sharing technology and expertise.

The challenge for rich citizens (whether in rich or poor countries) is to transform themselves from unthinking consumers to conscious global citizens. They must recognise their responsibilities and then make efforts to reduce their own individual carbon footprints. And they must urge their leaders to take the necessary long-term policy decisions and compensate the victims of climate change.

Finally, the time frame of global leaders is the next 18 months. This is the time left until the 192 nations that are party to the UN Framework Convention on Climate Change meet in Copenhagen to agree a successor to the Kyoto Protocol.

Between now and the meeting in December 2009, the presidents, prime ministers and government ministers must agree on the elements of a fair and equitable global pact that will address climate change over the two time frames mentioned above. This means both mitigating climate change by rapidly reducing greenhouse-gas emissions and adapting to the impacts ahead.

To succeed, our leaders must lose their current mindset of striving for the best deal for their own country and its citizens. They are not just representing their own countries, but all of humanity. They are negotiating the state of the world their generation will leave behind for their children and grandchildren.

The solutions lie in the hands of the leaders of all nations, especially those of the leading economies that emit most of the greenhouse gases that cause the problem. But citizens, especially those in the richer countries and rich citizens of the poorer countries, also bear a responsibility for action both personally as well as politically.

Our leaders must rise to the challenge and all conscious citizens must urge them to do so.

Saleemul Huq is head of the climate change group at the International Institute for Environment and Development. He is a coordinating lead author of the latest reports of the Intergovernmental Panel on Climate Change.