How much can I help reduce CO2 emissions from buildings?

“You can achieve amazing progress if you set a clear goal and find a measure that will drive progress toward that goal. This may seem pretty basic, but it is amazing to me how often it is not done and how hard it is to get it right.”

— Bill Gates, 2013 Annual Letter from the Bill and Melinda Gates Foundation

Recently, I spoke with two friends about the climate impact of buildings. After I explained to them the opportunity for buildings to mitigate climate change, they asked me how large of an impact I could make through this opportunity. Would it be meaningful? Conceptually, they supported me. However, they challenged me to go further, look deep into the numbers, and quantify my thinking. Doing so would help me clearly define the problem, set a clear goal, find a good measure, and — just maybe — achieve amazing progress.

Here’s my back of the envelope calculation. Below, I review the overall goal to mitigate climate change and the measure of progress against that goal. Then I estimate my potential contribution and compare it to the measure of progress.

The Overall Goal and Measure of Progress

Let’s begin with the overall goal for mitigating climate change. The Intergovernmental Panel on Climate Change’s (IPCC) recent report, Global Warming of 1.5 °C, explains why warming should be limited to no greater than 1.5 °C above pre-industrial levels. Every bit of warming matters. At 1.5 °C adverse impacts and risks are projected to be high in some regions and ecosystems. Also, we still have a chance to limit warming to within 1.5 °C, rather than the previously established 2.0°C goal. Therefore, a 1.5 °C increase should be avoided.

Limiting warming depends on limiting greenhouse gas emissions. Carbon dioxide (CO2) emissions are primarily what must be limited. As of 2011, about 1,900 gigatons of CO2 (GtCO2) from anthropogenic sources have been emitted as reported by the IPCC’s Fifth Assessment Report. Based on this amount, a carbon budget, the amount of allowable CO2 emissions remaining before reaching 1.5 °C, has been established.

The carbon budget shown in the table below is our measure of progress. Relative to the cumulative CO2 emissions reviewed during the 2006–2015 time period, to have a 2-in-3 chance of limiting warming to 1.5 °C, an additional 420 GtCO2 can be emitted. For worse odds, a 50/50 chance, a greater amount of CO2 emissions is allowable at 580 GtCO2. For even worse odds, a 1-in-3 chance, 840 GtCO2 can be emitted.

The carbon budget from the IPCC’s “Global Warming of 1.5 °C”

How the Building Sector Measures Up Against the Carbon Budget

Knowing the carbon budget, let’s now look at how fast we are spending it. How much CO2 are we emitting per year? In the Fifth Assessment Report, the total anthropogenic greenhouse gas emissions in 2010 was 49 GtCO2-eq (see the figure below). The suffix “-eq” means other greenhouse gases, such as methane, are included in the number. Of the total amount of emissions, the Buildings sector represented 19% or 9.18 GtCO2-eq, where indirect CO2 emissions from electricity use accounted for 6.02 GtCO2-eq.

From the IPCC’s Climate Change 2014 Synthesis Report

Improving the Building Sector Through Better Energy Use

In Global Warming of 1.5 °C energy, infrastructure, appliances, urban planning, transport, and adaptation options are available to facilitate the change needed to decarbonize our cities. Of these options, energy savings for buildings and its potential for CO2 reduction have been reviewed extensively by the IPCC. As we saw in the previous section, indirect CO2 emissions from electricity use account for two-thirds of the Buliding sector’s emissions. Let’s look deeper into the energy option then by reviewing current energy use and associated carbon emissions.

Chapter 9 of the Fifth Assessment Report, “Buildings”, reports the global energy consumption for residential and commercial buildings, respectively, was 24.3 petawatt hours (PWh) and 8.42 PWh in 2010 (see the figure below).

From Chapter 9 “Buildings” of the IPCC’s Fifth Assessment Report

According to the US Energy Information Administration, in 2017 the average annual electricity consumption for an American home was 10,399 kilowatt hours (kWh).

Generating electricity has an associated carbon cost, mostly from fossil fuel burning. In 2017, the United States emitted on average 1,009 lbs of CO2 for every megawatt-hour of electricity consumed (lbs-CO2/MWh). Depending on where electricity was consumed, this number varies. Vermont emitted the least at 16 lbs-CO2/MWh while Wyoming emitted the most at 2084 lbs-CO2/MWh.

Putting It All Together

In the above sections, I gathered information about the current goal to mitigate climate change, how we are currently performing against that goal, and how one opportunity—better energy use by buildings — is currently performing. Using these numbers, I can begin to estimate and compare my contribution.

Let’s recap:

Our current goal to mitigate climate change

  • We want to limit warming to 1.5 °C above pre-industrial levels

Our measure of progress against that goal

  • At 1.5 °C warming, the amount of allowable CO2 emissions remaining is about 420 GtCO2.

How are we performing against our goal?

  • In 2010, total anthropogenic greenhouse gas emissions was 49 GtCO2-eq.
  • The Buildings sector represented 19% or 9.18 GtCO2-eq, where indirect CO2 emissions from electricity use accounted for 6.02 GtCO2-eq.
  • In 2010, residential buildings consumed 24.3 PWh of energy.
  • In 2017, the average annual electricity consumption for a US home was 10,399 kWh.
  • The United States emitted on average 1,009 lbs-CO2/MWh.

Improving building energy use is a great opportunity for reducing CO2 emissions. To accomplish this requires a mix of solutions in technology, design, and policy. For a back of the envelope calculation, let’s consider the net zero energy building solution. From the “Buildings” chapter in the Fifth Assessment Report, net zero energy buildings refer to “building with on-site renewable energy systems (such as PV, wind turbines, or solar thermal) that, over the year, generate as much energy as is consumed by the building”. By creating buildings whose energy consumption, is net zero we are eliminating the CO2 emissions of that building’s energy use.

For simplicity, let’s consider only residential buildings and energy use from electricity where one building is equal to the average US home for electricity consumption 10,399 kWh.

Now, how many buildings could I work on over the course of my career? One major US developer, Toll Brothers, has sold 100,000 homes in 50 years. Rocky Mountain Institute has worked on over 1,000 buildings (see Footnote 3 in the linked paper), of which likely included larger and commercial buildings. This factor inevitably is highly variable and can change my estimate by an order of magnitude. Therefore, its use in this calculation is more indicative of the creativity needed to improve my potential impact. Let’s use 100,000 homes for now.


  • The energy use of a home per year
  • The average CO2 emissions per energy consumed
  • The number of homes worked on

I could contribute 0.0005 GtCO2/year or 0.001% of the 2010 global greenhouse gas emissions.

Is 0.001% a lot? Is it accurate? Perhaps it is a lot considering it’s from one person and his network. What if 1,000 people decided to do just the same? That would bring us to one percent, mathematically. However, here are some considerations: Not every building will be net zero. Not every building will be built at the same time. Carbon emissions from electricity generation vary from state to state. Home energy use varies above and below the average. Other building types — commercial, multi-family housing, etc — were not considered in this calculation.

The actual result matters less, I think, than its influence on me to think creatively on how to make this number as high as possible. Also, I think this result does provide an estimate on at least the potential order of magnitude of impact. It is not entirely insignificant, even with very rough estimates. For example, the result was not 0.0000000001%. And this makes me optimistic.

Special thanks to my friend Katie Patrick who wrote a great book called How to Save the World, which helped me think through how to approach this exercise.

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