A picture is emerging of how climate change will affect the St. Lawrence River valley and eastern Ontario – indeed, how it already has.

Data collected in both the U.S. and Canada shows that the St. Lawrence region of Eastern Ontario, Western Quebec and Northern New York is becoming hotter and, on average, wetter.

The U.S. government's National Climate Assessment, released last week, showed the New York side of the St. Lawrence Seaway as a hotspot of warming. It is one of a handful or areas in the continental United States where average temperatures in the past two decades are more than two degrees Fahrenheit above the average for the years 1901-1960.

The U.S. study did not look at data from the Canadian side of the river, but climatologist Jessica Spaccio of the Northeast Regional Climate Center at Cornell University in Ithaca, N.Y. says weather does not respect manmade frontiers.

"I think these results would be transferable to the Canadian side, just over the border."

The future climate of New York, said Spaccio, "may be something like what we have in Virginia today. You can see that shift northward."

Warmer Winter nights

Spaccio said the data shows that average temperatures have not increased uniformly throughout the year. It's the winters that have seen the greatest changes.

WEA Canada 20140106

Traffic makes its way along a puddle laden Somerset Street in Ottawa on Jan. 6, 2014. Climate data for the region shows Ottawa's winter nights are, on average, considerably warmer than a century ago. (Sean Kilpatrick/Canadian Press)

Those findings are confirmed by data collected by the Ontario Centre for Climate Impacts and Adaptation Resources, based at Laurentian University in Sudbury. Director Al Douglas says data shows an average temperature increase in the Ottawa area of just under one degree Celsius since the first half of the twentieth century. But that’s not the whole story.

"If you picked it apart a little further you'd find that it's actually the winter temperatures that are contributing more to the annual average temperatures. And to even get a little more specific it's the nighttime temperatures, the winter lows, that are really increasing the most."

Data collected from the Ottawa CDA station at the Experimental Farm near Dow’s Lake shows a huge 3.9 C increase in average winter lows between 1890 and 2010. Average summer highs rose by only 0.4 C over the same period.

Data from the Ottawa Airport weather station shows an increase of 2.5 C in mean winter lows since 1939, and no increase at all in summer highs.

The Ottawa data also shows that more precipitation is falling. The airport station shows an increase of 142 mm in total annual precipitation since 1939 — an increase of about 17 per cent. The experimental farm station registered a smaller increase of about 6 per cent.

Douglas says the reason is simple: hotter air can hold more moisture.

But the increase is not seen in the winter months. Precipitation at both Ottawa weather stations has decreased in the winter months, while rising or staying steady in the other seasons.

Art DeGaetano, director of the Northeast Regional Climate Center at Cornell, said there is also typically less snow on the ground than there used to be. "It’s consistent at stations right across New York and northern New England. The number of days in an average year with at least one inch of snow on the ground is down by about thirty."

Extreme weather on the rise?

Although climatologists agree on the major trends in the region, there is not as much consensus when it comes to the issue of drought and extreme rainfall.

While summer averages have changed the least, some climatologists warn averages can mask an increase in extreme events.

“What we’re noticing in summer is the number of very hot days," said Jessica Spaccio. "Maybe the average temperature isn’t changing that much, we’re going to see an increase in the number of heat waves. We classify that for the Northeast as three consecutive days over 90 F.  We’re going to see those lasting longer, and maybe an increase in intensity as well, so not just 90 F but maybe a bit hotter."

Hotter weather increases evaporation, which can lead to drought, said Spaccio. "But then we’ll have these extreme precipitation events. It’s kind of the worst of both worlds."

"It’s the number of days of extreme heat," said Al Douglas, "and the fact that the temperatures don’t really cool off at nighttime, so the heat really does remain."

Al Douglas says as much as 40 per cent of a year’s total precipitation could fall in just ten rainfall events.

"You have to consider the extremes, and that’s a pretty sensitive one for the agriculture sector."

Cornell's DeGaetano is more cautious in his assessment.

"It’s definitely warmer and there is more precipitation, but I don’t see a clear sign yet in the data that we’ll have longer dry periods followed by heavier rain," said DeGaetano.