Part I: Time Value of Carbon

when you save matters

what you build matters

what you don't build matters more

 by Larry Strain, FAIA

INTRODUCTION 

Climate change is time critical. If we continue with business as usual, global temperatures are predicted to rise 2°C above preindustrial times by 2030; this temperature change is widely accepted by the world scientific community as the point at which climate change becomes irreversible and catastrophic, often referred to as the global tipping point. We are about half way there, the climate has warmed by about 1° C. In 2013, the International Panel on Climate Change (IPCC) ran a number of emissions scenarios and only one kept us below 2°C: That scenario had emissions peaking by 2020 and fossil fuels phased out by 2055. When we evaluate emission reduction strategies, there are two things to keep in mind: the amount of reduction, and when it happens. Because emissions are cumulative and because we have a limited amount of time to reduce them, carbon reductions now have more value than carbon reductions in the future. The next couple of decades are critical. This paper focuses on emissions from the built environment and strategies to reduce them, particularly on embodied rather than operating emissions.

Figure 1. Emissions Scenarios

 The following terms are used in this paper:
Carbon, Emissions and Greenhouse Gas emissions are used interchangeably and all refer to Green House Gas emissions (GHG) which are made up of Carbon Dioxide (CO2) and other GHG’s, all of which are expressed as Carbon Dioxide equivalents (CO2e).
Embodied Carbon (eCO2): GHG emissions from materials and construction.
Operating Carbon (oCO2): GHG emissions from building operations — heating, cooling, lighting, plug loads.

BUILDING EMISSIONS
The built environment as an end user of fossil fuels is responsible for more emissions than any other sector. These emissions include emissions from building operations, (including electricity generation) and embodied emissions from materials and construction.

While constructing and operating buildings is responsible for almost half of U.S. GHG emissions, it also offers significant opportunities for reducing those emissions. The current gold standard for reducing emissions from buildings is to build new, net zero energy (NZE) buildings — very efficient, buildings powered by renewable energy sources, where the energy generated is equal to the energy needed to operate them. Because we build a lot of buildings, this is a critical piece of getting to a carbon neutral built environment. But there are two problems with relying on this strategy alone — building all of those new buildings will generate a lot of emissions and most building emissions come from less efficient existing buildings.

Figure 2. Consumption by End User Sector

Note: although energy use and GHG emissions are not the same, on a national scale, percentages for energy consumption and GHG emissions from buildings are roughly equivalent.
 
We need strategies that can produce large savings quickly, and because some reduction strategies result in an initial increase in carbon emissions from materials and construction — we need strategies that can produce net reductions within the next critical 10-30 years. Ultimately, we will need a built environment that is carbon neutral.

Ideally, all new buildings should be net zero energy (and emissions), but once buildings have eliminated operating emissions, two other sources of emissions become more important in the short term:

1. Embodied emissions from building materials, and construction processes.
2. Operating emissions from the existing buildings we already have.

NEW BUILDINGS: The importance of embodied carbon emissions (eCO2)

When we started to really pay attention to energy efficiency after the first global energy crisis in the 1970s, we were focused on saving energy, not reducing GHG emissions, and embodied energy and their associated emissions were generally ignored. This was because over a building’s lifespan, typically 75–100 years, embodied emissions only accounted for 10%-20% of a building’s total emissions. But a couple of things have changed since then: GHG emissions have become more critical than energy; and as buildings have become more efficient and operating emissions have dropped, embodied emissions now make up a much larger percentage of total lifetime emissions. Embodied emissions are also important because of when they occur—they are the first emissions from a new building. When a building is constructed—before it starts operating and generating operating emissions—it is already responsible for tons of GHG emissions. And even though the majority of embodied emissions happen once—when the building is constructed—and operating emissions happen over time and are cumulative, the majority of GHG emissions for the first 15 – 20 years of a building’s life will be the embodied emissions from materials and construction. If we succeed in making new buildings net zero energy (NZE), then the only emissions will be the embodied emissions. In the long run, it’s still important that new buildings be NZE, but in the short term we need to focus on reducing embodied emissions.

This is not a simple thing to do. We know how to make NZE buildings, but it is much more difficult to reduce embodied emissions to zero. There are immediate steps we can take—reducing the quantity of the materials in our buildings and selecting materials with lower carbon footprints—but modern, industrial materials generate significant GHG emissions in their production. Ultimately the modern material economy will need to become a carbon neutral material economy. 

(to be continued)

To read the entire paper, Time Value of Carbon, click here.