A major new report from the Intergovernmental Panel on Climate Change has expressed with greater certainty than ever before that human influences are the dominant cause of climate change and that they are growing.
It has warned of irreversible and dangerous impacts with severe and pervasive effects on people and ecosystems unless stringent efforts to reduce carbon emissions are implemented.
The IPCC said the level of carbon dioxide in the atmosphere is at levels not seen in at least 800,000 years.
It said the greenhouse gasses that result from human activities are the dominant cause of climate change.
IPCC Chairperson Rajendra Pachauri said the scientific case for prioritising action on climate change is clearer than ever.
This is the starkest message ever about climate change from the scientific community to governments and policymakers all over the world.
Having sifted through 30,000 new scientific papers over the past six years, more than 800 lead authors and editors, 1,000 contributing authors, 2,000 expert reviewers, and scientific experts from 195 governments have agreed that it is extremely likely that human activities are driving climate change.
They have warned that the more human activity disrupts the climate, the greater the risks of severe, pervasive and irreversible impacts for people and ecosystems.
The report synthesised and summarised almost 5,000 pages of scientific findings produced by the three IPCC working groups over the past 14 months.
It found that the levels of three key greenhouse gases - carbon dioxide, methane and nitrous oxide - are the highest in at least 800,000 years.
It said from 1880-2012, the global average surface temperature rose by 0.85 degrees Celsius, while the global mean sea level rose by 19cm from 1901-2010.
Mr Pachauri said the world has "little time before the window of opportunity to stay within 2C of warming closes.
"To keep a good chance of staying below 2C, and at manageable costs, our (carbon) emissions should drop by 40 to 70% globally between 2010 and 2050, falling to zero or below by 2100."
The UN panel said global economic growth "would not be strongly affected" by rolling back carbon emissions, but the cost will rise if action is delayed.
Strong message coming from @IPCC_CH - human influences are dominant cause of climate change but there's an opportunity to mitigate effects— Will Goodbody (@willgoodbody) November 2, 2014
The panel has warned that continued emissions of greenhouse gases will cause further warming, as well as long lasting changes in all aspects of the climate system.
It said that many of the risks constitute a particular challenge to the least developed countries and vulnerable communities.
People who are socially, economically, culturally, politically, institutionally or otherwise marginalised are especially vulnerable to climate change, according to the report.
The IPCC said that limiting the effects of climate change raises issues of equity, justice and fairness.
Youba Sokona, Co-Chair of the IPCC working group, said it is technically feasible to transition to a low carbon economy, but appropriate policies and instruments are lacking.
He said the longer we wait to take action, the more it will cost to adapt and mitigate climate change.
Mr Pachauri said that we have the means to limit climate change.
The solutions are many and allow for continued economic and human development, he said, but the will to change is what is needed.
The report will form the scientific basis for governments and policymakers to negotiate a new global climate change agreement in Paris next year.
It represents the final input that scientists will have into that agreement.
Former president Mary Robinson, who heads a climate justice foundation, has said the science of climate change is clear.
She said: "We cannot say we do not know the facts.
"Transformative leadership from all levels of community, business and politics can drive world leaders to commit to brave and ambitious actions; the task of shaping an appropriate global response to climate change lies in their hands.
"Climate action makes sense and the opportunities it creates are ours for the taking."
Minister for the Environment Alan Kelly said Ireland "must do everything" in its power to reduce emissions.
He said: "We must move as quickly as possible to a low carbon climate resilient economy. We have already started this process and must progress with haste."
Campaign group Friends of the Earth has called for action to reduce Ireland's contribution to climate change.
Spokesman Oisin Coughlan said the Government can begin by introducing the necessary legislation to kick-start a meaningful change of policy.