The unusually hot spells and extreme drought that North America and other parts of the world have experienced over the last few years are inextricably linked to global warming, a prominent NASA scientist claims in new study.

Prof. James Hansen is a scientist at NASA's Goddard Institute for Space Studies in New York and a professor at Columbia University. His paper, Perception of Climate Change, will be published next week in the Proceedings of the National Academy of Science.

"Combined with the well-established fact that global warming is a result of increasing atmospheric carbon dioxide and other greenhouse gases, it follows that the increasingly extreme climate anomalies are human-made," Hansen's study claims.

Hansen says his study is based on statistics, not the more typical climate modelling, and that it was written before the record-breaking temperatures the U.S. has seen this year.

Hansen's position was supported by a noted Canadian scientist in a leading British newspaper on Sunday.

In an interview in the Independent, Prof. Andrew Weaver said the science in Hansen's study is excellent "and reframes the question."

A climate scientist at the University of Victoria in British Columbia, Weaver was a member of the Nobel Prize-winning international panel of climate scientists that issued a series of reports on global warming.

"Rather than say, 'Is this because of climate change?' That's the wrong question. What you can say is, 'How likely is this to have occurred with the absence of global warming?' It's so extraordinarily unlikely that it has to be due to global warming," Weaver is quoted as saying.

In his paper, Hansen says "some additional warming will occur over the next few decades. However, we can slow, halt and reverse global warming over a period of several decades. The key requirement is to stop subsidizing fossil fuels and to begin to collect a fee from fossil fuel companies in proportion to the amount of carbon they are putting into the air."