By Alex Kirby BBC News Online environment correspondent

The choices this generation makes will be crucial for our descendants, according to a United Nations report.

Geo-3... is the most authoritative assessment of where we have been, where we have arrived, and where we are likely to go

Dr Klaus Toepfer, Unep

Published by the UN Environment Programme (Unep), established 30 years ago, the report details some real improvements since then.

But it says the overall trend is adverse, especially in poor countries.

By 2032, it predicts a planet likely to have been largely affected by human hands.

The report is Unep's Global Environment Outlook-3 (Geo-3), the work of more than 1,000 authors.

It records some significant achievements since the 1972 Stockholm environment conference which led to Unep's establishment.

Not fast enough

In North America and Europe there have been improvements in river and air quality. The international effort to halt the chemical damage to the ozone layer is another success, with recovery to pre-1980 levels likely by mid-century.

Disasters do more harm

But generally, the report says, there has been a steady environmental decline, especially in much of the developing world. It says this is increasing people's vulnerability to natural hazards like cyclones, floods and droughts.

Geo-3 says: "There is a growing gap between rapid rates of environmental degradation and the slow pace of social response.

"The evidence suggests that many areas of the world are on trajectories that will lead them into crisis, and that little time is left for creating effective responses."

Marked progress

Dr Klaus Toepfer, Unep's executive director, said: "Geo-3 is neither a document of doom and gloom, nor a gloss over the acute challenges facing us all.

Planetary stress

Population pressure There are 2,220m more people alive today than in 1972 Soil decline Around 2bn ha of soil, 15% of the Earth's surface, is now classed as degraded by human activities Water About half the world's rivers are seriously depleted and polluted. Serious water shortages were affecting 40% of the world's people by the mid-1990s Forests Since 1990 they are estimated to have declined by 2.4% Wildlife Nearly 25% of mammal species and 12% of birds are regarded as globally threatened Fish Just under a third of global fish stocks are defined as depleted, over-exploited, or recovering from over-fishing

"It is the most authoritative assessment of where we have been, where we have arrived, and where we are likely to go."

Dr Toepfer told BBC News Online a lot had changed for the better in Unep's 30 years.

"Willy Brandt used to demand 'blue sky over the Ruhr'", he said.

"It is blue now. So Europeans can feel there's been real action, even if security, health, globalisation and immigration have now moved close to the top of their agenda."

Given the wrong decisions today, within 30 years we could be living on a drastically impoverished planet, Unep believes.

By then, it says, more than half the world's people could be living in areas of severe water stress.

Unep says hunger can be beaten

More than 70% of the Earth's land surface could be marked by the impact of cities, roads, mining and other human developments.

Yet Geo-3 is emphatic that the future does not have to be like that. By 2032, it thinks, the proportion of hungry people could be just 2.5% of the world's population.

Levels of carbon dioxide, which many scientists think is intensifying natural climate change to dangerous levels, could be starting to stabilise.

Common fate

The report examines four scenarios which it says "tell strongly contrasting but plausible stories" about how the world might develop.

They are:

• Markets First, where the industrialised world's values prevail through market-driven developments
• Policy First, where governments take strong action to reach specific goals
• Security First, "a world of great disparities, where inequality and conflict prevail"
• Sustainabilty First, a world with a new model of development, and more equitable values and institutions.

"We need concrete actions, concrete timetables, and an iron will. It cannot be the responsibility of politicians alone. We are all shareholders in this enterprise."