As the Rio+20 Earth Summit kicks off Monday in Rio de Janeiro, one high-profile academic is arguing countries should be assigned an environmental rating, much the same way financial agencies rate government credit. His proposed ranking for Canada? Fifth out of the G20 countries.

"The finance industry has used similar techniques to rate the economic performance of nations for many years," writes ex-Oxford University professor Matt Prescott. "Our new environment ratings could provide a useful tool for investors seeking to reduce their exposure to both short and long-term risks."

Prescott has just released preliminary environmental ratings for 20 of the world's major economies, known collectively as the G20.

Canada received a BBB+ rating, putting it fifth behind Germany, the UK, France and the U.S. Prescott's report cited Canada's "energy efficient power stations, plenty of fresh water and low levels of corruption" for its relatively high performance.

"Unfortunately," it noted, "Canada is seriously let down by its very low provision for marine protected areas and very high CO2 emissions per capita."

Did Prescott go too easy on Canada? BBC News correspondent Richard Black speculates that Canada's rating may "raise some eyebrows in the environmental community," particularly for its massive bitumen industry.

Then there's also Stephen Harper government's Bill C-38, which raised such an uproar that more than 400 environmental groups and others recently blacked out their websites in protest.

A separate ranking report released last week by Corporate Knights, a Toronto-based quarterly, concluded that B.C. and Ontario are leading Canada in environmental performance, while Alberta and Saskatchewan sit right at the bottom.

Geoff Dembicki reports on energy and climate change for The Tyee.