Human actions have been the dominant cause of climate change, leading to the warming up of the atmosphere and oceans, melting of snow and ice and the rise in the global mean sea level, says a new assessment by the Intergovernmental Panel on Climate Change (IPCC) Friday.
The fifth assessment report (AR 5) released here says global warming in the climate system is unequivocal and since 1950 many changes have been observed throughout the climate system that are unprecedented over the decades to the new millennia.
The report said each of the last three decades has been successively warmer at the earth's surface than any preceding decade since 1850.
"It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century," the report highlighted.
The first part of the report deals with the physical science - atmosphere, ocean, ice among others - basis of climate change. During the next one year, the panel will release two more parts of the report based on thousands of peer-reviewed studies.
"Our assessment of the science finds that the atmosphere and oceans have warmed, the amount of snow and ice has diminished, the global mean sea level has risen and the concentrations of greenhouse gases have increased," said Qin Dahe, co-chair of IPCC Working Group I.
Thomas Stocker, the other co-chair, said: "Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system. Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions."
"Global surface temperature change for the end of the 21st century is projected to be likely to exceed 1.5 degree Celsius relative to 1850 to 1900," said Stocker.
Projections of climate change are based on a new set of four scenarios of future greenhouse gas concentrations and aerosols, spanning a wide range of possible futures.
"This provides important insights into the scientific basis of climate change. It provides a firm foundation for considerations of the impacts of climate change on human and natural systems and ways to meet the challenge of climate change," said IPCC co-chair Rajendra Pachauri.
The report projects that heatwaves are very likely to occur more frequently and last longer while wet regions would receive more rainfall, and dry regions less.
"As the ocean warms and glaciers and ice sheets reduce, the global mean sea level will continue to rise, but at a faster rate than we have experienced over the past 40 years," said Qin Dahe.
The report further says as s a result of past, present and expected future emissions of CO2, "we are committed to climate change, and effects will persist for many centuries even if emissions of CO2 stop".
The report was prepared by 209 lead authors and 50 review editors from 39 of the 195 member countries and more than 600 contributing authors from 32 countries.
On the controversy over the report saying the rate of warming over the past 15 years (1998-2012) is smaller than the rate calculated since 1951, it said: "Due to natural variability, trends based on short records are very sensitive and do not in general reflect long-term climate trends."
Civil society organisations called for the governments and policy makers to act decisively on the climate crisis.
"This report shows that the science on climate change is clear. The debate about who is responsible is over. It is time that governments take action to address this issue in an effective manner, and not through half-hearted efforts," said Sanjay Vashist, director of Climate Action Network South Asia.
Voicing similar views, Greenpeace called for action by government on climate change.
"The only logical response to a warning of this magnitude is immediate action (by government," said Jai Krishna, Climate and Energy Campaigner, Greenpeace India.
