The UN climate change conference, to be held in Copenhagen in December, should provide the climax to two years of international negotiations to reach a new treaty aimed at addressing the causes and consequences of greenhouse gas (GHG) emissions.

A global deal on climate change, to succeed the Kyoto Protocol that expires in 2012, is urgently needed. Concentrations of carbon dioxide (CO2) and other GHGs in the atmosphere have reached 435 parts per million (ppm) of CO2-equivalent, compared with about 280 ppm before industrialization in the 19th century.

If we continue with business-as-usual emissions from activities such as burning fossil fuels and cutting down forests, concentrations could reach 750 ppm by the end of this century. Should that happen, the probable rise in global average temperature, relative to pre-industrial times, will be 5 C or more.

The last time the earth's temperature was that high was more than 30 million years ago. The human species, which has been around for no more than 200,000 years, would have to deal with a more hostile physical environment than it has ever experienced.

Floods and droughts would become more intense and the sea level would be several meters higher, severely disrupting lives and livelihoods, and causing massive population movements and inevitable conflicts around the world. Some parts of the world would be under water; others would become deserts.

Developing countries recognize and are angered by the inequity of the current situation. Existing GHG levels are largely the result of industrialization in the developed world.

Yet developing countries are the most vulnerable to the consequences of climate change, which threaten the economic growth that is necessary to overcome poverty. At the same time, emissions cannot be reduced to the required level without the central contribution of the developing world.

Climate change and poverty, the two defining challenges of this century, must be tackled together. If we fail on one, we will fail on the other. The task facing the world is to meet the environment's "carbon constraints" while creating the growth necessary to raise living standards for the poor.

To avoid the severe risks that would result from a rise in global average temperature of more than 2 C, we must get atmospheric concentrations below 450 ppm. This will require a cut in annual global emissions from about 50 gigatonnes of CO2-equivalent today to below 35 gigatonnes in 2030, and less than 20 gigatonnes by 2050.

Today, the per-capita annual emission in the European Union (EU) and the US is 12 tonnes 23.6 tonnes, compared to 6 tonnes in China and 1.7 tonnes in India. Since projections for 2050 suggest that the world's population will reach about 9 billion, the per-capita emission has to be cut to about 2 tons of CO2-equivalent, on average, if we want to keep the global annual total to less than 20 gigatonnes.

Most developed countries are targeting reductions in annual emissions of at least 80 percent - relative to levels in 1990 - by 2050. If they are to convince developing countries that the 2050 goal is achievable, they must be ambitious and realistic both about the domestic political challenges they face in adopting and implementing the demanding targets for 2020, 2030 and 2040.

Developing countries need substantial help and support from rich nations in order to implement their plans for low-carbon economic growth, and to adapt to the effects of climate change, which are now inevitable, over the next few decades. Developed countries should also provide strong support for measures to halt deforestation in developing countries and for reducing emissions substantially, quickly and at a reasonable cost.

Based on recent estimates of the developing world's extra requests as a result of climate change, rich countries should be providing annual financial support - in addition to existing foreign-aid commitments - of about $100 billion for adaptation and $100 billion for mitigation by the early 2020s.

Some of the latter can come through carbon trade. Rich countries must also demonstrate that low-carbon growth is possible by investing in new technologies, which should be shared with developing countries to boost their mitigation efforts.

We are already seeing extraordinary innovations by the private sector, which will drive the transition toward a low-carbon global economy. Investments in energy efficiency and low-carbon technologies, too, could pull the global economy out of its economic slowdown over the next couple of years.

More importantly, in driving the transition to low-carbon growth, these technologies could create the most dynamic and innovative period in economic history, surpassing that of the introduction of railways, electricity grids or the Internet.

There is no real alternative. High-carbon growth is doomed, crippled by high prices for fossil fuels and killed off by the hostile physical environment that climate change will create. Low-carbon growth will be more energy-secure, cleaner, quieter, safer and more bio-diverse.

We should learn from the financial crisis that if risks are ignored, the eventual consequences are inevitably worse. If we do not start to combat the flow of GHG emissions now, the stock in the atmosphere will continue to grow, making future action more difficult and costly. Other public expenditure can be postponed, but delaying climate change measures is a high-risk, high-cost option.

Climate change poses a profound threat to our economic future, while low-carbon growth promises decades of increased prosperity. The choice in Copenhagen will be stark, and the stakes could not be higher. We know what we must do, and we can do it.

The author is chair of the Grantham Research Institute on Climate Change and the Environment and professor of Economics and Government at the London School of Economics and Political Science. He is former head of the UK's Economic Service and chief economist at the World Bank.