Climatologists must be objective and honest, says Met Office chief executive John Hirst, but they are also human

John Hirst is talking, with admirable patience, about the summer that never was. The now infamous Met Office press release, which declared the UK was "odds on for a barbecue summer", came back to haunt the forecaster when wet and blustery weather turned July into a washout. And there was further embarrassment to come. The Met Office projected a "milder than average" winter, before Britain suffered one of its coldest, snowiest spells in memory, with temperatures almost stooping to South Pole levels.

Hirst, the organisation's chief executive, says the "barbecue summer" press release was carefully "couched in probabilities" and was designed to discourage the media from colouring the forecast with inaccurate descriptions. Normally the newspapers tend to reach for their own superlatives, "scorcher" being a popular choice when prospects are good, but these interpretations are sometimes "completely conflicting," says Hirst, "so [with 'barbecue summer'] we were aiming for consistency."

The phrase proved a gift for headline writers. "It was well reported," agrees Hirst, "but when we got to July, which was very wet, all that deep understanding went out the window." It's worth noting, he says, that both the summer and winter forecasts were seasonal, a more inexact process than daily forecasting. Although the organisation has been providing seasonal forecasts for the last five years, says Hirst, "it's not part of our operational suite." It is, he says, a "developing science".

This of course begs the question: why is an experimental, unproven forecasting method being used as the basis for a national press release? "It's there as a point of interest. We do try to couch it carefully in probabilities. Sadly those probabilities get lost in the headlines." In any case, adds Hirst, the summer forecast was more or less right. "Objectively, we said 'warmer than average, drier than average'. The whole of the summer was warmer than average, it was just massively wetter in July than we had anticipated 120 days before."

It may be of little comfort to anyone who rushed out to buy a barbecue, but making seasonal forecasts in other parts of the world is often much easier. "We got the late onset of the monsoons in India, the number of hurricanes in north America, [and] the heavy rain that led to mudslides in northern Brazil. [The UK is] one of the most difficult areas in the world to forecast," says Hirst. "We sit on an island against a major continent, up against a massive ocean. The disruption that comes from the patterns in the weather is really quite complex."

Despite its success as a global forecaster, the Met Office's long-term forecasting has been severely criticised by rival weather companies and climate change deniers who claim that the organisation is attempting to make a case for global warming by consistently projecting warmer weather. Although this is an accusation it strongly denies, earlier this month the Met Office admitted to BBC News that its annual global mean forecast predicted higher than actual temperatures for nine years out of the last 10.

Does Hirst feel an obligation to make a case for climate change? He says it's not the Met Office's job to influence opinion. "We support people making policy on these things, but we don't design policy." Hirst says it's the organisation's role to "make sure the science is robust, clear and understood. And then people can draw their own conclusions." But, he adds, "if people are using inappropriate or faulty data we try and correct that."

Hirst is not above making his own view clear. "When I came to this job, I was sympathetic to the possibility of climate change, but not really understanding of it. Having seen the science displayed to me, and the professional impartiality of the scientists at this organisation, I have no doubts about climate change and of man's contribution to it, because that's what the science shows.

"Some people say to me, do you believe in climate change? Climate change is not a religious conviction. It is not a philosophy. It is not a question of metaphysics. It is a geophysical phenomenon that we observe scientifically."

The Met Office works closely with the University of East Anglia to record global temperatures, which are used in the reports of the Intergovernmental Panel on Climate Change (IPCC). Although recently leaked emails from the university's Climate Research Unit seemed to suggest an evidence bias towards climate change, Hirst strongly rejects the accusation that his fellow scientists manipulated the data.

"A scientist has an obligation to be objective and work with high integrity at all times," he says, adding: "Scientists are human, like everybody else. And they aren't perfect in their use of language. Somebody hacked in to some personal exchanges. We don't always, all of us, work on the basis that everything we say or write down is going to be made public and examined in fine detail. Otherwise none of us would say very much to anybody at all."

Climate change deniers often target the Met Office's seasonal forecasting as evidence of its inability to accurately predict changes in climate. But they are two separate issues, explains Hirst. Seasonal forecasts predict a specific outcome over the long term, whereas climate is about predicting an average over the long term. "When you move to climate, you're talking about general weather patterns on a very broad scale. In climate, you are not able to say that in June 2052 the weather will be like this, but you can say over the decade of the 50s it will be broadly this kind if shape. Some people say to me 'you can't get the season right so how can you get the climate right', but it's a different problem."

Hirst is keen to highlight the commercial benefits of accurate forecasting. Weather predictions can be used to make operations more efficient and more profitable, he says. "Look at fresh and chilled foods: quite a lot of that demand is influenced by the weather patterns. But not many people make sophisticated use of the forecasts to make them more efficient. What it needs is a bit of investment to establish the correlation between demand and weather patterns."

But what if retailers stocked up on barbecue items ahead of a rainy summer, or lighter clothing ahead of an unexpected cold snap?  "If you are Starbucks and planning your next week's pattern of purchasing, knowing the next week's weather is pretty helpful, you don't need a seasonal forecast. At M&S they have a six-day, two-day and six-hour decision point on their lettuce provision: that's well within the very accurate [short-term] forecasting parameters."

As a government trading fund, required to operate on a commercial basis, the Met Office offers an enormously wide range of services, from advising utilities companies—"Some of the power companies' profits can swing up to £600,000 a day depending on whether they can forecast the temperatures correctly"—to North Sea operations—"if you're towing an oil rig you need a three-four day window of calm weather"—to health services, such as its partnership with primary care trusts that helps sufferers of chronic obstructive pulmonary disorder. "The disease is aggravated by cold weather," explains Hirst.

Hirst accepts the regular criticism that comes with the job as a function of our national fixation with the weather. When there's "a lot of weather about", he says, "our press [officers] are hoarse by the end of the day. People ask me, why did I take this job? Well the answer is, I'm British and by definition obsessed by the weather."

He says the strain of being "under the microscope all of the time" is tempered by the knowledge that his organisation is providing a "fantastic" service. "Most private sector [companies] would kill for the scientific understanding and intellectual property we have in this organisation. You have to take the bumpy bits along with the joy of doing something really meaningful," he says.