The costs of inaction on climate change will be "catastrophic", according to US Secretary of State John Kerry.
Mr Kerry was responding to a major report by the UN which described the impacts of global warming as "severe, pervasive and irreversible".
He said dramatic and swift action was required to tackle the threats posed by a rapidly changing climate.
Our health, homes, food and safety are all likely to be threatened by rising temperatures, the report says.
Scientists and officials meeting in Japan say the document is the most comprehensive assessment to date
of the impacts of climate change on the world.
In a statement, Mr Kerry said: "Unless we act dramatically and quickly, science tells us our climate and our way of life are literally in jeopardy. Denial of the science is malpractice.
"There are those who say we can't afford to act. But waiting is truly unaffordable. The costs of inaction are catastrophic."
Rajendra Pachauri, chair of the Intergovernmental Panel on Climate Change (IPCC), which produced the report, told BBC News: "Even in rich countries, the impacts of climate change could lead to greater incidents of pockets of poverty, even in rich countries could lead to impoverishment of some particular communities.
"However there is an equity issue, because some of the poorest communities in the poorest countries in the world are going to be the worst hit."
Some impacts of climate change include a higher risk of flooding and changes to crop yields and water availability. Humans may be able to adapt to some of these changes, but only within limits.
An example of an adaptation strategy would be the construction of sea walls and levees to protect against flooding. Another might be introducing more efficient irrigation for farmers in areas where water is scarce.
Natural systems are currently bearing the brunt of climatic changes, but a growing impact on humans is feared.
Members of the IPCC say it provides overwhelming evidence of the scale of these effects.
The report was agreed after almost a week of intense discussions here in Yokohama, which included concerns among some authors about the tone of the evolving document.
This is the second of a series
from the UN's climate panel due out this year that outlines the causes, effects and solutions to global warming.
This latest Summary for Policymakers document highlights the fact that the amount of scientific evidence on the impacts of warming has almost doubled since the last report in 2007.
Be it the melting of glaciers or warming of permafrost, the summary highlights the fact that on all continents and across the oceans, changes in the climate have caused impacts on natural and human systems in recent decades.
In the words of the report, "increasing magnitudes of warming increase the likelihood of severe, pervasive and irreversible impacts".
"Nobody on this planet is going to be untouched by the impacts of climate change,'' said Mr Pachauri.
Dr Saleemul Huq, a convening lead author on one of the chapters, commented: "Before this we thought we knew this was happening, but now we have overwhelming evidence that it is happening and it is real."
Michel Jarraud, secretary-general of the World Meteorological Organization, said the report was based on more than 12,000 peer-reviewed scientific studies. He said this document was "the most solid evidence you can get in any scientific discipline".
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