A leading British climate scientist has told the BBC he believes the target to limit global warming to 1.5C will be missed.

Professor Sir Bob Watson, former head of the UN climate body, told the BBC’s Today programme he was “pessimistic”.

His warning comes amidst a summer of extreme heat for Europe, China and the US.

The UN says passing the limit will expose millions more people to potentially devastating climate events.

The world agreed to try to limit the temperature increase due to climate change to 1.5C above pre-industrial levels at a UN conference in Paris in 2015. That target has become the centrepiece of global efforts to tackle climate change.

Climate scientists have been warning governments for years that they are not cutting their countries’ emissions quickly enough to keep within this target.

But it is surprising for someone as senior and well respected as the former head of the UN climate science body the IPCC to be so frank that he believes it will be missed.

Professor Sir Bob Watson is currently Emeritus Professor of the UK’s Tyndall Centre for Climate Research – having previously worked at the UN, Nasa, UK’s Department of Environment and the US White House – and is perhaps one of the foremost climate scientists in the world.

In the interview aired on Thursday he said: “I think most people fear that if we give up on the 1.5 [celsius limit] which I do not believe we will achieve, in fact I’m very pessimistic about achieving even 2C, that if we allow the target to become looser and looser, higher and higher, governments will do even less in the future.”

Although his comments are candid on the state of action on climate change, many of his colleagues will agree with his conclusion that we are on course for a temperature rise of 2.5C or more. Based on current government commitments to cutting greenhouse gas emissions Climate Action Tracker predicts that global temperatures will rise to 2.7C .

The figure is not a direct measure of the world’s temperature but an indicator of how much or how little the Earth has warmed or cooled compared to the long-term global average – and even slight changes can have significant impacts.

The UN climate body, the IPCC, has said keeping temperature rises below 1.5C, rather than 2C, would mean:

Prof Sir Bob Watson said that the world is struggling to prevent temperature rises as we are not reducing emissions fast enough.

“The big issue is we need to reduce greenhouse gases now to even be on the pathway to be close to 1.5C or 2C. We need to reduce current emissions by at least 50% by 2030. The trouble is the emissions are still going up, they are not going down,” he said.

He told the Today programme that setting targets was not enough and countries needed to back these up with action: “We need to try and hold governments to start to act sensibly now and reduce emissions, but even governments with a really good target like the United Kingdom don’t have the policies in place, don’t have the financing in place to reach those goals.”

In March the UK’s watchdog on climate change said the UK had lost its leadership on climate issues. It said Government backing for new oil and coal, airport expansion plans and slow progress on heat pumps showed a lack of urgency across government on the issue.