Temperatures, sea levels, acid levels in oceans and ice sheets were already moving "beyond the patterns of natural variability within which our society and economy have developed and thrived," scientists said in a report released Thursday.
The findings came at the end of a three-day conference in Copenhagen, Denmark, where nearly 2,000 researchers gathered to discuss climate change.
The group called on policy-makers to use all tools available to reduce dangerous emissions of greenhouse gases.
The current climate situation on the planet may be as severe as the worst-case scenarios predicted by the Intergovernmental Panel on Climate Change (IPCC), which issued warnings in 2007 of a future beset by flooding, drought, storms and mass extinction of species.
In its report, the researchers also warned of potential social costs across the planet because of climate change.
Temperature rises above 2 degrees Celsius would lead to climate disruption for the rest of the century and disproportionately affect poor nations, the researchers warned.
"Recent observations show that societies are highly vulnerable to even modest levels of climate change, with poor nations and communities particularly at risk," the report said.
The conclusions of the conference will be presented to politicians when they meet in Copenhagen in December. It is then that leaders will discuss a new global agreement on greenhouse gas emission levels to replace the Kyoto Protocol, which expires in 2012.
