If you had $1 million dollars and your goal was to reduce as much CO2 in the atmosphere as possible by 2050, what would you spend it on? Buy a bunch of carbon credits? Protect a piece of the Amazon? Buy everyone in your neighborhood an electric car Oprah style? “YOU get a car, and YOU get a car…”

Effective altruism is a form of charity that asks: how can we use our resources to help others the most? The answers must be based on data about outcomes, not emotions. Applying effective altruism to climate change and the question becomes: how can we use our resources to most reduce climate change? In his book “The Most Good You Can Do”, the father of effective altruism Peter Singer suggests:

If your goal were solely to slow down climate change by reducing greenhouse gas emissions, you could do that more effectively by donating to organizations that are encouraging people to go vegetarian or vegan than by donating to leading carbon-offsetting organizations.

Ok, so donating to a company that encourages people to go vegetarian is a cheaper cost per ton of CO2 avoidance than buying a one ton carbon offset. Interesting… But what about other alternatives like buying solar panels, promoting biking, buying an EV?

Luckily, I’m not the first person to think of this idea. I found an organization called DrawDown.org that estimates the CO2 reduction potential of different technologies as well as the cost of implementing that technology. DrawDown was founded by environmentalist Paul Hawken and looks like it has a team of 8 full-time researchers with science related PhDs. On their solutions leader board they show the top 80 CO2 reducing technologies including predictable technologies like solar and wind to less predictable ones like bamboo, a “substituted for aluminum, concrete, plastic, or steel.” Here’s the top 10 technologies by total CO2 reduction:

If you sum the total CO2 reduction by sector, you find that the highest potential sectors to decarbonize are electricity generation, food, land use, materials, women & girls, buildings/cities, and transport, in that order.

Drawdown.org summary of CO2 potential by sector

Sources of CO2 by sector from 2014 IPCC Report

What’s interesting is that the CO2 reduction potential by sector is not a perfect match with the CO2 emissions by sector. Above are the sources of CO2 by sector as reported by IPCC in 2014. If you overlay the two graphs you’ll notice that transport is significantly under-represented with 4% CO2 reduction potential versus 14% actual global emissions. Food and land use, on the other hand, are 46% of total CO2 reduction potential versus 24% actual global emissions. The takeaway for me is that transportation is going to be a challenging sector to decarbonize, whereas there are more opportunities for food and land use.

What we are still missing is COST. The right question to ask is not how much CO2 reduction potential is there, but rather which sectors offer the best CO2 reduction per dollar spent. The good news is that Drawdown calculated this figure for 55 of the 80 solution types. Unfortunately, for 6 of the top 10 solutions solutions they wrote “ GLOBAL COST AND SAVINGS DATA TOO VARIABLE TO BE DETERMINED”. I understand that academics hold themselves to a certain level of scientific rigor before publishing any estimates. But certainly we should be able to do some napkin math to get an idea of the magnitude differences between solutions: $10 billion? $100 billion? $1 trillion? $100 trillion?

Since I am not held to the same level of scientific rigor, I did napkin math for the missing 25 solution types here (all feedback welcome). The purpose of my calculations was not to appear in a peer reviewed scientific journal, but rather to get a rough estimate of the cost. Here are two examples:

Reduced Food Waste: 35% of food is thrown away in high-income economies. Changing people’s habits is difficult. Another public awareness campaign that had to change people’s habits was anti-smoking. Estimates of the anti-smoking campaign cost per quitter was $480. Therefore, if we multiply $480 by the high-income economy population, we can have a rough estimate of changing food waste behavior via a public awareness campaign. There are 1.22 billion high-income citizens x $480/person = $586 billion.

Plant Rich Diet: In economics there is a concept known as price elasticity of demand which measures how much demand for a product decreases if price increases. Meat, for example, has a price elasticity of 0.7. Therefore, if the price of meat increased 10%, demand would decrease by 7%. To estimate the price increase needed to reduce global meat consumption by 50%, we could calculate what the price increase would have to be: 50% / 0.7 = 71% price increase. The global meat market is about $1 trillion dollars, so reducing meat consumption 50% would cost $710 billion per year.

Once we have the estimates of cost, we can divide the cost by the CO2 reduction potential to get cost per Gigaton (GT) of CO2. When you view the technologies by cost per GT of CO2 avoidance, the picture completely changes. X-axis is the amount of CO2 reduction and y-axis is the cost per GT (both axes on log scale). The green area marks solutions that have high impact and are also cost effective. The yellow areas are either cost effective or high impact. The red box shows solutions that are both expensive and not very impactful. Keep in mind that only cost was considered so any savings/revenue generated by these solutions was not considered. For example, wind and solar would generate electricity revenue.