Blair's comments came in a UK government-commissioned report published on Monday that collates evidence presented at a conference on climate change, hosted by Britain's Meteorological Office last year.
The report says scientists now have "greater clarity and reduced uncertainty" about the impacts of climate change.
In a foreword, Blair said it was clear "the risks of climate change may well be greater than we thought".
"It is now plain that the emission of greenhouse gases, associated with industrialisation and economic growth from a world population that has increased sixfold in 200 years, is causing global warming at a rate that is unsustainable," he wrote.
Over the next century, global warming is expected to raise ocean levels, intensify storms, spread disease and shift climate zones, possibly making farmlands drier and deserts wetter.
Tipping pointThe UN-backed Intergovernmental Panel on Climate Change says temperatures rose by 0.6 degrees Celsius during the 20th century.
Computer modelling predicts increases of between 2.5 degrees and 1.4 degrees and 5.8 degrees Celsius by the year 2100, depending on how much is done to limit greenhouse gas emissions.
Scientists have warned of climatic tipping points such as the Greenland and west Antarctic ice sheets melting and the Gulf Stream shutting down.
"The risks of climate change may well be greater than we thought"
In the British report, the head of the British Antarctic Survey, Chris Rapley, said the huge west Antarctic ice sheet may be starting to disintegrate, an event that could raise sea levels by five metres.
Rapley said a previous report from the panel that played down worries about the ice sheet's stability should be revised.
"The last IPCC report characterised Antarctica as a slumbering giant in terms of climate change," he wrote "I would say it is now an awakened giant. There is a real concern."
Blair's vow to put climate change at the centre of the international agenda during Britain's leadership of the G8 and the European Union last year met with limited success.
He was unable to overcome the Bush administration's antipathy to the Kyoto climate-change accord - rejected by the US government on the grounds it would damage the economy.
British ministers also have acknowledged that Britain is unlikely to meet its own target of cutting carbon dioxide emissions by 20% by 2010.