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Trees Are Great But They Cannot Achieve Carbon Neutrality Alone

William Hughes
May 17, 2022

Guidehouse Insights Sustainability

Elon Musk has offered a prize of $100 million in a carbon-capture contest. Some quipsters on the Internet mock this offer, suggesting that we simply plant trees. Even if this suggestion is meant to be ironic and amusing, it shows a lack of understanding of the scale of global carbon emissions and the rate and level of decarbonization that is required to meet Paris Agreement targets. Forestation alone cannot solve the problem. It is a classic case of not seeing the forest for the trees. 

The Scope of the Problem

In February, the U.S. Energy Information Administration published projections to 2050 in its Annual Energy Outlook 2021 report. Data in the report shows that energy-related CO2 emissions generated about 4.8 Gt (billion metric tons) in just 2020. 

The good news is that this amount is declining. Quoting the International Energy Association, “US emissions are now down almost 1 Gt from their peak in the year 2000, the largest absolute decline by any country over that period.” A reduction is wonderful, but it is useful to put this amount in perspective. 

One way is to produce an order of magnitude estimate about the amount of space that would be needed to capture 4.8 Gt of carbon emissions. According to the Urban Forestry Network, “On average, one acre of new forest can sequester about 2.5 tons of carbon annually. Young trees absorb CO2 at a rate of 13 pounds per tree each year. Trees reach their most productive stage of carbon storage at about 10 years at which point they are estimated to absorb 48 pounds of CO2 per year.”

When you divide 4.8 Gt by 2.5 imperial tons per acre, the math works out that the US would need to plant 2.1 billion acres of new forests. At 640 acres per square mile, 3.3 million square miles of new forests would need to be planted to get to net zero carbon. To put this in perspective, the area of the US, including the continental US, Alaska, and Hawaii, is only 3.70 million square miles. 

If we were to scale this amount back and wait for the trees to be at their most productive stage of carbon storage, the US would need to plant on 900,000 square miles of currently unforested land. Alaska is 663,267 square miles, and the 35% of the state that is forest is about all that can grow there. Most of the state is not suitable for growing trees. 

In addition, tree planting on a large scale for carbon offsets is problematic. It requires that forest cannot even be harvested in perpetuity in addition to the challenges of biodiversity and property rights. 

Keeping It Real

It is important to maintain a sense of humor about Internet quips, and acknowledge that reforestation is a good solution from many perspectives. However, it would be unfortunate if anyone was to be misled into thinking that trees are the only solution necessary to achieve carbon neutrality given the acreage needed. The reality is that decarbonization requires many solutions and encouraging innovation with a significant cash reward should be commended.