Assessing green infrastructure as an effective strategy to help cities to build resilience to climate change

The increase of greenhouse gases (GHG) emissions in the atmosphere due to human activities such as fossil fuel burning, deforestation and land-use changes, is causing increases in surface temperature. From the pre-industrial era to the current days, the carbon dioxide (CO₂) concentration has increased from 280 ppm to 398.17 ppm (NOAA, 2015) and according to the Intergovernmental Panel on Climate Change (IPCC, 2014) globally averaged combined land and ocean surface temperature data show a warming of 0.85°C per decade over the period 1880 to 2012. Under higher temperatures, the atmosphere has a higher capacity to hold water vapor (Horton et al, 2010), increasing the time interval between rain events and the magnitude of precipitation especially in extreme events. Thus, at higher temperatures the frequency at which precipitations occur tends to decrease while the intensity of such precipitations tends to increase. In the urban environment, where typically the majority of surfaces are impermeable, the impact of climate change tends to exacerbate the occurrence and the intensity of floods as well as droughts and heat waves. Within this context, there has been much discussion about strategies that could effectively help cities to reduce their CO₂ emissions to the atmosphere (mitigation strategies ) and to adapt to the impacts caused by climate change (adaptation strategies ). Regarding the impacts caused by urban floods, a decentralized approach, known as green infrastructure (GI) has been proposed as an alternative to the traditional concrete infrastructures (gray infrastructures [GR]). GI sustains, or attempts to replicate pre-development site hydrology in the post-development condition (Montalto, 2007), taking advantage of natural processes like infiltration, interception and evapotranspiration to manage stormwater (Davis et al, 2012). Beside capturing precipitation and reducing the amount of runoff that is convened to the sewer systems, GI can provide other benefits such as reduction of heat island effects, increased air and water quality, carbon sequestration, expansion of recreational spaces, increased habitat for flora and fauna among others (Wise et al, 2010). Because of their capacity to deliver multiple benefits, GI has been proposed as a sustainable alternative for cities to mitigate and adapt to climate change (Mason & Montalto, 2014; Union European, 2010). Several government grants have been launched recently to focus on the development, application and evaluation of methodologies for integrating GI into urban spaces as adaptation efforts to climate change (DOI, 2014; NOAA, 2014). Nevertheless, the body of literature that assesses GI as an effective strategy to help cities to build resilience to climate change remains small. For instance, the performance of designed urban green spaces under climate change is still poorly understood. In addition, the comparison between potential benefits of GI applied to urban watershed scale with the environmental costs associated with their installation and maintenance is still poorly supported by research (Pataki et al 2011). In order to better explore these research gaps, this thesis aims to evaluate GI as a means of reducing climate risks in the urban northeast environment. To reach out this main objective, we propose three different hypotheses: Hypothesis #1: Green infrastructure can reduce GHG emissions associated with urban drainage infrastructure · Compared to grey (stormwater management) infrastructure strategies, GI releases lesser GHG emissions over its entire useful life Hypothesis #2: GI can help cities adapt to extreme precipitation · GI facilities can significantly reduce urban runoff even during extreme precipitation Hypothesis #3: GI vegetation is not vulnerable to climate change, especially to floods and droughts · The risk of plant mortality within the expected envelope of climate variability (floods and droughts) is insignificant. The three hypotheses, as well as, a preliminary chapter that introduces the thesis topic, are presented separately in a scientific journal format. Chapter 1 reviews literature about the leading climate risks facing the Northeast Region (NE) of Unites States of America (USA), while provides an overview of the ongoing GI initiatives in the USA and their potential value for reducing vulnerability to the key climate risks faced by the urban northeast region. Chapter 2 addresses hypothesis #1 and includes a study conducted at the watershed scale level that used life cycle assessment techniques to compare the carbon footprint of a green and a grey strategy to reduce combined sewer overflow occurrence (CSO) in a highly urbanized watershed. Chapter 3 addresses hypothesis #2 via an investigation at the site scale that evaluated the performance of a bioretention installed in an urban watershed during extreme events including Hurricane Sandy and Hurricane Irene. Chapter 4 addresses hypothesis #3 and presents an experiment conducted in a greenhouse that evaluated the response of two species commonly used in vegetated GI sites to consecutive periods of floods and drought. This thesis finalizes with a general conclusion section for all the chapters.

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