Over the Autumn and Winter there have been a fair few posts here about sustainability in design and architecture and about planning to mitigate climate change.

Articles about both sustainability and about the impact of severe weather from the changing climate have become much more prominent in the press, reflecting public concern, and in January a major Danish architecture award - the Arne Prize - went to the climate mitigation works around Sankt Kjelds Plads in the city.

It's not all grim news. The average carbon footprint for someone living in Denmark has dropped from over 14 tonnes a year in 1995 to about 6 tonnes a year now, twenty five years later, but there is still a long way to go. Statistics indicate that Denmark has the fifth highest household carbon footprint in Europe and those statistics may well get worse before they get better. A recent article in the magazine Wired looked at a new problem because, with cheap and sustainable energy from wind farms, international data centres are considering a move to Denmark and, as they are notoriously greedy on energy, that could increase the carbon footprint for the country by as much as 10% by 2030 and there have been articles in national newspapers about a scandal over carbon offset schemes that now appear to have been fraudulent.

But I realised that I had absolutely no idea just what a tonne of carbon dioxide looks like or, come to that, how much carbon could be offset by planting a tree.

Then I came across the photograph of a balloon outside the parliament buildings in 2007 that represents a tonne of carbon. And suddenly I had the image of walking around Copenhagen this time next year followed by six of these and my trips to the recycle bins with bags of plastic food containers and glass jars seemed a bit feeble

Photosynthesis removes carbon dioxide naturally and trees are good at storing carbon removed from the atmosphere by photosynthesis but a young tree absorbs about 1.4 kilos (3lbs) of carbon dioxide in a year and when more established and larger, so after about 10 years of growth, a tree can absorb 22 kilos (48lbs) of carbon dioxide a year.

An acre of mature trees absorbs about 3 tonnes of carbon a year so Danes would need just over 2 acres of woodland each to absorb their personal carbon footprint.

If you can't visualise an acre, then if you planted the garden area of Sankt Annæ Plads with trees all the way from just behind the equestrian statue of Christian X at the end towards Bredgade and down as far as the water at the edge of the harbour, that would be about 2 acres so would just about do it …. for you. So where do the trees for the rest of your family go …. or, come to that, the trees for the other million or so people living in Copenhagen?