Time is running out to minimize the risk of setting in motion irreversible and long-term climate change and other dramatic changes to Earth's life support system, according to scientists speaking at the Planet Under Pressure conference.

The unequivocal warning is delivered on the first day of the four-day conference opening with the latest readings of Earth's vital signs.

In subsequent days at the meeting, nearly 3,000 experts spanning the spectrum of interconnected scientific interests, will examine solutions, hurdles and ways to break down the barriers to progress. The conference is the largest gathering of experts in development and global environmental changes in advance of June's UN "Rio+20" summit in Brazil.

"The last 50 years have without doubt seen one of the most rapid transformations of the human relationship with the natural world," says speaker Will Steffen, a global change expert from the Australian National University.

"Many human activities reached take-off points sometime in the 20th Century and sharply accelerated towards the end of the century. We saw a 'Great Acceleration'."

"It is the scale and speed of the Great Acceleration that is truly remarkable. This has largely happened within one human lifetime."

Key indicators of the planet's state, according to the speakers: higher levels of carbon dioxide in the atmosphere, phosphorus extraction and fertilizer production causing many large dead zones in coastal areas; rising air and ocean temperatures; melting sea ice, polar ice sheets and Arctic permafrost; rising sea levels and ocean acidification; biodiversity loss; land use changes; and growing consumption of freshwater supplies and energy by a growing global population, of which billions of people still lack even the most basic elements of well-being.

At a planetary level, humanity is altering the global carbon cycle, water cycle and nitrogen cycle, says Professor Steffen. Indeed, humans now produce more reactive nitrogen artificially than all natural processes on land.

"Where on Earth are we going?" he asks, underlining several potentially dangerous environmental "tipping points" foreseen, among them the melting of the polar ice sheets and the thawing of perennially frozen northern permafrost soils.

Current research estimates the permafrost alone stores the equivalent of roughly twice the carbon in the atmosphere, he says. Under a "high warming scenario," projected releases of greenhouse gas emissions from melting Arctic permafrost are the equivalent of 30-63 gigatonnes (Gt) of carbon dioxide by 2040, 232-380 Gt by 2100, and 549-865 Gt by 2300. By comparison, fossil fuel emissions today are roughly 10 Gt per year.

"The key point is: Either we turn around a lot of these trends - the carbon dioxide trend, deforestation and so on - or we allow them to continue and push beyond critical thresholds."

"There are signs that some drivers of global change are slowing or changing," says fellow speaker Professor Diana Liverman, co-Director of the Institute of the Environment at the University of Arizona and visiting Oxford University academic.

"Population growth is slowing and will level off; the intensity of energy and carbon required for a unit of production is declining; agricultural intensification is slowing and forests are starting to expand in some regions."

"On the other hand, average resource consumption per person, already high in some regions, is growing steeply in emerging economies even as many poor people cannot meet basic human needs.

"In some countries people are consuming far too much, including carbon, water and other resources embodied in trade. We have a long way to go to turn things around."

Liverman notes a time lapse animation offering vivid evidence that Earth has entered a new geological epoch hallmarked by the profound ecosystem impacts of one species - humans - so much so that it marks an entirely new geological timespan: the "Anthropocene."

Online here, it illustrates the dramatic growth of carbon dioxide emissions from the start of the industrial revolution - spreading from the UK in 1750 across Europe, North America and to Japan by 1900.

"By the end of the 20th Century we have high emissions in China, India, Europe and eastern North America but relatively little across Latin America and Africa. Here lies the core of the debate about responsibilities for climate change in relation to historical and per capita emissions."

She also refers a recent UK study showing the highest income earners are responsible for three times the level of emissions compared with lowest income earners.

"In countries with high income inequality, the richest 10% of the population may be responsible for more than 50% of the greenhouse gas emissions - and the growing middle classes of many developing or transitional countries are developing consumption habits that add to the burden on the earth system."

Says conference co-chair and UNESCO director of the science-policy division, Dr Lidia Brito: "If you like, our presenters today are akin to doctors saying 'look, you may not feel too sick at the moment but you've got high blood pressure, your cholesterol is going up, and your lifestyle is not conducive to good health.'"

"There is time to turn these trends around and promising, more positive messages will be delivered by colleagues in days to come. We look forward to discussions of our most promising options, the barriers to change and to a prescription for the future."