the climate brief Accurate Reporting of Climate Science

In a recent BBC Radio 4 programme on the origins of the Centre for Alternative Technology (CAT), the founders discussed why, in 1973, they'd felt moved to set up an eco-centre in an old Welsh slate quarry. Long before renewables were anything approaching mainstream, CAT was building wind turbines out of canvas and wood, and solar panels out of old radiators.

Since then renewable technology has been on a rhetorical journey.  The sustainable "good life" dream of the CAT pioneers has given way to issues of economic viability in the twenty-first century "green economy". But is the shift in framing from good life to green economy a movement betraying its principles, or evolving to address contemporary issues and concerns?

From Good Life to green economy

CAT's mission, states Radio 4's Sue MacGregor, "was to promote alternatives to the modern, high polluting technologies that produce so much of our energy, food and buildings". CAT's founders use phrases like "limits to growth" and "how to blend science with a way of doing more with less, and working with nature rather than destroying it". It's the philosophy that inspired Surbiton couple Barbara and Tom's pursuit of the sustainability dream in 1970s sitcom The Good Life.

Despite greenhouse gas emissions continuing to rise fairly steadily, earth's surface - that's the land and top of the oceans - has warmed relatively slowly over the past fifteen years or so.

We asked climate scientists to give us their thoughts about what's causing the recent slower pace of surface warming. Here's what they told us.

Recent slower warming isn't unusual

To draw conclusions about climate change, climate scientists tend to look at long time periods - temperatures measured over decades, or whole centuries. The last 15 years or so is a relatively short amount of time to measure temperatures over.

There are good reasons for taking a longer view. As professor  Gabriele Hegerl from the University of Edinburgh tells us, surface temperatures bounce around from one year to the next because of natural fluctuations in the climate:

"Climate change becomes more visible against [natural fluctuations when] the longer periods are considered ... To see the long-term change, long periods need to be considered, such as multiple decades."

The concentration of carbon dioxide in earth's atmosphere is poised to pass 400 parts per million. That might not sound like a lot, but it's nearly half as much again as pre-industrial levels, and higher than it's been for hundreds of thousands of years.

We asked scientists what the number means, and what we should take from the fact that this once distant prospect is now just around the corner.

Mauna Loa

On the side of a Hawaiian volcano sits the Mauna Loa Observatory, where scientists have been measuring levels of carbon dioxide in the atmosphere since the 1950's.

Other observatories around the world monitor carbon dioxide too. But the emblematic Keeling curve - named after C. David Keeling who started the measurements in 1958 - is the longest standing continuous record.

On May 1st, the instruments at Mauna Loa were recording 399.39 parts per million of carbon dioxide in the atmosphere - and media reports have suggested that we're about to pass the 400 ppm mark. Averaged over a month, the values are a bit lower. The latest monthly average for March is 397.34 ppm.

Will it be this year?

Plants and trees absorb carbon dioxide as they grow. This means concentrations of the gas rise and fall with the seasons, reaching a peak in May just before summer in the northern hemisphere.