Alongside other Commonwealth head of states, Cameron was originally unable to attend due to clashes with diamond jubilee celebrations for Queen Elizabeth II. Following the Brazilian governments decision to change the dates, foreign delegates are disappointed that the head of the Queen's government still isn't coming.
Despite pledging back in 2010 to lead "the greenest government ever," Cameron will instead send his deputy Nick Clegg, arguing that the Eurozone crisis is needs his attention.
"Cameron's claims to be the greenest government ever are just complete greenwash," observes Iain Keith from Avaaz.
"What's happening in the UK is just disastrous. They're gutting every single environment program we have and calling it 'austerity measures' and they're still giving subsidies to the fossil fuel industry," laments the Scotsman.
The point of the Rio Conference is not just to ensure the environment remains intact. It is also about building sustainable economies that don't crash leaving; millions unemployed.
Like the Earth Summit of 1992, Rio+20 aims to ensure development meets the needs of the present without compromising the ability of future generations to meet their own needs.
The debt crises Europe faces today are a result of corporate greed, poor regulation - in essence: a lack of sustainability. And yet Cameron believes that putting a bandaid on Greece's debt problems is the most pressing concern.
During his election Cameron promised his government would "drive the green economy to have green jobs," unlike others before.
Where better to promote such a goal than Rio+20, where one the main focus points is the "green economy" and driving "green jobs"?
Despite Cameron's absence, it will still be the largest UN conference ever, with over 50,000 people attending including 130 heads of state.
No part of running a government doesn't require sustainable development. Poor urban planning will be on show when London comes to a standstill during the Olympics. Global greenhouse emissions are already causing damage to oceans and to the climate. Unregulated mining and oil extraction is already destroying the livelihood of people native to the land. The global economy is catastrophically fragile and dependent on the whim of largely unregulated bankers.
These are the problems of today, not just tomorrow. Cameron's petty display of short-sightedness is not only an embarrassment to Britain, but a cold rebuke to the people ruined by the crises the world currently faces.
By Lachie McKenzie, photo by Moritz Hager, video by 10:10.
Follow Lachie McKenzie on Twitter: www.twitter.com/lachiemckenzie