Does Higher Temperature Increase Rain Intensity? Not Always, Observations Show Decreasing Rain Intensity. Southern Ontario Twice As Many Statistically Significant Decreases In Annual Maximum Rainfall.

One degree temperature rise increases water vapour holding capacity
by 7%, but does it increase rainfall intensity?

High school science teachers and media have been saying that temperature increases associated with climate change cause a direct increase in water vapour and therefore, by association, more extreme rainfall.  This has been reported for years, like here in the Guardian where they say "A warmer atmosphere can hold more moisture, and globally water vapour increases by 7% for every degree centigrade of warming."

The Clausius-Clapeyron (C-C)
equation describes the water-holding capacity of the atmosphere as a function of temperature.

Geophysical Research Letters research looks at historical data to see if this theory linking temperature and rain intensity can be verified and what other explanatory variables are available. Researches from Lamont-Doherty Earth Observatory, Columbia University, MIT, and Institute Centre for Water Advanced Technology and Environmental Research (iWater), Masdar Institute of Science and Technology, and Department of Chemical and Environmental Engineering, Masdar Institute of Science and Technology analyzed how extreme rainfall intensities in the USA depend on temperature (T), dew point temperature (Td), and convective available potential energy (CAPE). The analysis considers geographic sub-region, season, and averaging duration.

What did researchers find in the data?:

"When using data for the entire year, rainfall intensity has a quasi Clausius-Clapeyron (CC) dependence on T, with super-CC slope in a limited temperature range and a maximum around 25°C

So Clausius-Clapeyron is only quasi-valid, meaning there is not a strong relationship between rain intensity and temperature. And rain intensities peak at 25 degrees Celcius ... they do not keep going up with temperature increases. The Guardian missed these details. Who else made the temperature-water vapour-rainfall relationship claim:

The magazine Science article How Much More Rain Will Global Warming Bring? touches on the 1 degree - 7 % atmospheric vapour relationship back in 2007. Bloggers around the world repeat this, and even David Suzuki is saying it. But lets look at more the the research findings based on actual data in Geophysical Research Letters. These charts show how rain intensities do not increase at the CC rate above 22 degrees:

The fourth column of charts shows temperature T on the x-axis. On the y-axis is slope of the relationship between rain intensity and temperature. The dashed red line is the predicted CC rate, meaning above 22 degrees rain increases less that predicted by CC. So no, this theory does not hold water (pun intended). In fact for some of the highest temperatures for some quantiles in the North Central and South, slope is negative, meaning that increased temperature DECREASES rainfall intensity (black lines go below zero).

Looking at the third column of charts with LnP on the y-axis, we see that for several quantiles of precipitation in both winter and summer, LnP does not reach the predicted rate at all (coloured lines below the predicted rates shown in the black dashed lines). In plain english this means the predicted increase in rain intensity with temperature is never met for small storms, e.g., the ones responsible for erosion, etc. So the theory is flawed for small storms.

In the summer, i.e., black lines in third column, precipitation as LnP flattens out or sometimes decreases at the highest temperatures, mostly in the South and Central of the US - for the lower 2 to 3 quantiles the CC rate is not met or just met. In the North, rain intensity for the lower quantiles of precipitation flattens out and decreases above 25 degree Celcius.

The take-away is that simple relationships make great theories. Real systems are more complicated than the Clausius-Clapeyron (CC) would suggest.

Lets look at something simpler in Ontario, Canada. Temperatures have increased. At right are temperature trends plotted by Statistics Canada. There is an increase from the late 1940's to 2008. Pretty clear.

Below are maximum annual observed rainfall trends for Toronto's long term climate station from Environment and Climate Change Canada's Engineering Climate Dataset Version 2.3, from the 1940's to 2007. It shows decreasing annual maximum rainfall for all rainfall durations from 5 minutes to 24 hours. Obviously the real world data shows us that despite increasing temperature, there is no corresponding increase in maximum observed rainfall.

The hypothesis that rising temperatures result in higher water vapour and then also more extreme rainfall is rejected based on the observations in southern Ontario. While temperatures are up in Ontario, there are twice as many statistically significant decreasing annual maximum rainfall trends as increasing ones as summarized from the Engineering Climate Dataset (version 2.3):

More statistically significant DECREASES in rainfall intensity are observed than increases.

For short duration rainfall, the convective storms that cause flash flooding in urban areas, we can look at the duration of 2 hours or less - there is just one statistically significant increase in annual maximum rainfall, and 6 examples of statistically significant decreasing rainfall maximum.

Evidence-based policies for flood mitigation and other stormwater or water resources management activities first require accurate characterization of factors affecting runoff and flow conveyance in municipal and natural drainage systems. By hypothesizing that rainfall intensities are increasing as a result of higher temperatures, flood damage mitigation could be achieved by combating green-house gas emissions to stall temperature increases. Data shows that extreme rainfall is not increasing with temperatures, and therefore an increase in flood damages is due to other factors (e.g., hydrology, hydraulics) - as a result effective flood damage mitigation must focus on key drivers and not temperature or rainfall trends.

We cannot explain severe weather, extreme rainfall, tornados and hail in Ontario with simple relationships that have been shown to contradict observation data.

***

Canadian data analyzed by researchers at the University of Western also concluded that the Clausius-Clapeyron (C-C) equation did not match real temperature and rain data as observed in climate stations including Vancouver, Brandon, London and Moncton. As shown on the following graphs the real data relationships (coloured lines) do not follow the theoretical C-C scaling lines (dashed lines).

For Vancouver, precipitation decreases at higher temperatures (downward sloping solid lines).

For Brandon, London and Moncton, the slope of the precipitation-temperature trend line is less than the theoretical dashed line for most positive temperatures. In Moncton the trend is flat, meaning higher temperatures above 5 degrees C do not increase precipitation.

Key conclusions of the Western analysis were:

"Summary
- The sub-daily daily maximum precipitation shows weak linear correlation to the daily temperature for most stations and durations. Only lower durations for Moncton, London and Brandon show correlations roughly identical to the theoretical C-C 7% per 0C rate.
- For Vancouver station none of the sub-daily durations present linear correlation to temperature. For temperatures higher than 10 ºC negative slopes are observed.

Conclusion
- The Clausius-Clapeyron scaling rate clearly does not apply for any of the stations consider in this study, and should not be arbitrarily applied to derive IDF curves for future."

The analysis was presented at the ICLR Friday Forum in March 2017.

Researches also concluded that the use of Western's IDF_CC tool projections of future IDF would be preferred to any reliance on the C-C equation and its theoretical 1 degree = 7% scaling factor.

Environmental Impacts of Green Infrastructure Construction - CO2 Emissions for Soil Removal and Aggregate Extraction & Transportation, Ontario Impacts

Etobicoke Infiltration System - Green Infrastructure for
Stormwater Infiltration and Low Impact Development

Like any infrastructure that requires material resources and energy to construct, green infrastructure is no different than grey infrastructure. This post looks at a typical low-impact-development (LID) feature for stormwater runoff control and estimates the CO2 impacts of its initial construction, excluding traffic impacts during construction, and excluding impacts for ongoing operation and maintenance, or rebuilding at end of lifecycle. Emissions are then scaled up across Ontario in light of green infrastructure policies being considered province-wide. The GHG emissions are huge for building green infrastructure !

For this example, consider a perforated pipe and gravel trenches for infiltration of stormwater runoff, like the Etobicoke Infiltration system pioneered in pre-amalgamation City of Toronto. This is a typical green infrastructure configuration - see Ryerson University's Planning and Design Manual at this link. We can assume a simple arrangement with the perforated pipe and gravel trench beyond the roadway, say in the boulevard.

Napkin - engineer's friend ... especially when
they run out of envelopes to scribble on the back of.

The construction of the perforated pipe infiltration system requires transport of various materials to and from the construction for initial construction. Lets assume we are considering 1 cubic metre of infiltration storage volume in the system. Some activities and their resulting kilometre-tonnes of transportation are estimated on this blog-napkin, if you will, aka an educated Fermi Estimate:

1) excavation, transport and disposal of native soil material - if the facility is in a retrofit setting the sodium adsorption ratio of the soil (due to road salt chloride accumulation over time) means reuse could be limited. Assume the disposal site may be a distance of 40 km from the construction site. For each cubic metre of infiltration storage, lets assume a 40% voids ratio in the gravel infiltration trench, meaning each cubic metre needs 1/0.4 - 1.2 cubic metres of soil disposed, and at a density of about 2 tonnes per cubic metre. So 40 x 1.2 x 2 = 96 km-tonnes of native soil haulage.


2) transport and placement of clear stone - assume aggregate comes from a quarry 40 km away from the site. For each cubic metre of infiltration storage, and our 40% voids ratio in the gravel, each cubic metre of storage needs 1/0.4 - 1.2 cubic metres of clear stone. At a density of 1.6 tonnes per cubic metre (3/4 inch clear stone), there is 40 x 1.2 x 1.6 = 76.8 km-tonnes of gravel haulage.

The OECD indicates pollution for truck transport in a report that indicates 140 grams of CO2 emitted per tonne-km. So adding 1 +2 above, a total of about 173.8 km-tonnes of green infrastructure construction to achieve 1 cubic metre of runoff storage results in 173.8 x 0.14 = 24.3 kg of CO2 emissions.

3) quarrying gravel - based on 4.32 kg CO2 emitted per tonne from this source, each cubic metre of infiltration storage with 40% voids requires 1/0.4 - 1.2 cubic metres of clear stone. At a density of 1.6 tonnes per cubic metre again, there is 4.32 x 1.2 x 1.6 = 8.29 kg CO2 per cubic metre of storage.

The MOECC is targeting 25 mm or more of green infrastructure storage and/or treatment which means per hectare of urban development with 50% rain/runoff there is 10,000 x 0.025 x 0.5 = 125 cubic metres of runoff and green infrastructure storage needed. So 125 x 24.3 = 3037.5 kg of CO2 emitted per hectare of urban area retrofitted with this low impact development feature. That is just haulage. To quarry the gravel adds 8.29 x 125 = 1036.8 kg per hectare. Total is 4074.3 kg per hectare of green infrastructure runoff treatment. Is that big?

Ontario has 825,000 hectares of untreated urban area.

Given the Ontario-wide 852,000 hectares of urban area that would need green infrastructure retrofits under the draft MOECC policy, the Ontario-wide CO2 emitted to initially build green infrastructure would be 852,000 x 4074.3 = 3,471,303,600 kilograms of CO2 emitted. Say 3.5 million tonnes of CO2 added.

So over 3 billion kilograms to build green infrastructure to manage runoff - you could argue there is some nominal climate mitigation offset-benefit if there is some type of vegetated pre-treatment filter before the perforated pipe / gravel infiltration. If so, that would be offset by the frequent inspection and minor maintenance visits by municipal crews, year after year.

Note in Canada the per capital emission was 20.1 tonnes CO2 equivalent in 2015 according to Environment and Climate Change Canada. So retrofitting green stormwater infrastructure in Ontario will emit CO2 equivalent to all the CO2 emitted by 172,000 Canadians over an entire year. That is just initial construction.

So that is just a neat Fermi Problem on green infrastructure to consider along the the other considerations on cost, cost-effectiveness, impacts to existing infrastructure, and need for scientifically-based local targets for green infrastructure. Currently, MOECC is considering blanket green infrastructure retrofit targets that do not consider any cost constraints or any no environmental impacts of construction like CO2 emissions. Earlier posts note that impacts to iron watermains and flood prone sanitary sewer systems was ignored. Let's hope a more holistic approach emerges that recognizes that we can't build millions of tonnes of infrastructure, green, grey or purple, and not have big impacts to emissions etc.

Green infrastructure for stormwater management, just like green energy for power supply has significant negative impacts. It should not be viewed as a panacea for urban stormwater and water resources management issues in Ontario. Check out this post on costs and other impacts.