科學家再確認：地球越來越熱 正跨越無可逆轉的臨界點

Scientists at the Planet under Pressure conference in London today added their stern warning to others issued over the past several weeks - time is running out to minimize the risk of irreversible, long-term climate change and other dramatic changes to Earth's life support system.

The 2,800 scientists, policymakers and business representatives opened their four-day conference with a reading of Earth's vital signs and an ominous prognosis, "without immediate action, societies everywhere face an uncertain future on what may become a much hotter planet."
Lake Hume, the furthest downstream of the major reservoirs on Australia's Murray River system (Photo by Tim Keegan)

Hosted by The Royal Society UK's Living with Environmental Change program, this is the largest gathering of experts in global sustainability ahead of the UN Rio+20 summit in Brazil in June and the largest gathering ever of such a group of experts.

To Professor Will Steffen, a conference speaker, there are several potentially dangerous environmental "tipping points," among them the melting of the polar ice sheets and the thawing of perennially frozen northern permafrost soils.

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

Steffen calls the "explosion in human activity" over the past several decades, "The Great Acceleration."

"Many human activities reached take-off points sometime in the 20th century and sharply accelerated towards the end of the century. It is the scale and speed of the Great Acceleration that is truly remarkable," Steffen said. "This has largely happened within one human lifetime."

"Where on Earth are we going?" he asks.

Key indicators of the planet's state, conference speakers agreed, include growing consumption of freshwater supplies and energy by swelling numbers of people worldwide, even as billions of people lack even the most basic elements of well-being.
Professor Will Steffen (Photo by TEDx Canberra)

There are higher levels of carbon dioxide in the atmosphere. Phosphorus extraction and fertilizer production send nutrient runoff into the sea, causing huge dead zones in coastal areas.

Then the scientists report, we have rising air and ocean temperatures, melting sea ice, polar ice sheets and Arctic permafrost, rising sea levels and ocean acidification, biodiversity loss and land use changes.

At a planetary level, humanity is altering the global carbon cycle, water cycle and nitrogen cycle, warns Steffen. He worries about the release of the greenhouse gas carbon dioxide from melting permafrost because it stores the equivalent of twice the carbon in the atmosphere.

Steffen warns about the "compost bomb," another potential contributor to a hotter Earth. The microbial respiration in thawed soils, he says, are "leading to a tipping point where heat is produced more rapidly than it can be dissipated."
Permafrost on the Yukon Delta National Wildlife Refuge, Alaska (Photo by USFWS Alaska)

"All these environmental tipping point phenomena are part of a single system," that Steffen says becomes clear, "when we look at how the Earth has behaved in the past."

"The key point is," he said, "we may reach a threshold for the Earth as a whole this century. 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 the Earth as a whole across a threshold whereby a lot of these tipping elements are activated and the world moves into a new, much warmer state."

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

Liverman says 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 that she calls the "Anthropocene."

Pressure will ease off the planet somewhat, says Liverman. "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," she said, "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."

The 1:10 minute time-lapse history of human global CO2 emissions, online at www.youtube.com/watch?v=MEMse22h8c8, captures the growth of CO2 emissions from their start with the UK's Industrial Revolution in 1750 and radiating across Europe, to North America, then Asia and worldwide.