The Long-Term Goal We Need: The Science of Climate Neutrality

We’re halfway through the two weeks of the climate change negotiations here in Paris, and one contentious part of the draft text being negotiated is Article 3.1, entitled “Collective Long-Term Goal.” This will be a fundamental to the Paris Agreement, because it will establish what the nations of the world agree to be their ultimate objective in terms of global warming. Will it be to limit it to 1.5 degrees Celsius above the pre-industrial average, or 2 degrees, or—God forbid—no limit at all?

But beyond the number, there’s another important scientific issues being debated in this section. It’s expressed in three concepts that are part of the draft text, but “bracketed”—not yet agreed to. They’re “[climate neutrality]”, “[decarbonization]” and “[carbon budget]”; there’s also a statement of the goal as “[Achieving zero global GHG emissions by 2060-2080]. What do these words mean, how are they related, and why are they so important?

First, a bit of scientific background. There are several greenhouse gases (global warming pollutants), but the most important by far are carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). There are also different sectors of the global economy that emit these gases. The energy sector (including electric power, transportation, industry) plus cement production mostly  emits CO₂. The land sector—agriculture and forests—emits CO₂ as well, mostly from deforestation, but is dominated by emissions of methane (e.g. from cattle and from rice paddies) and of nitrous oxide (from fertilizers and manure).

Besides the predominant gases emitted, there’s another critical difference between the energy the land sectors. That’s the fact that the land sector can not only emit CO₂, e.g. as forests are cut down, but can also take CO₂ out of the atmosphere as they grow back. In negotiating language we say that the land sector carries out both “emissions” and “removals.” Another way of expressing this is that in net terms, it could be either a “source” (emissions are greater than removals) or a “sink” (emissions are less than removals.)

As Nancy Harris of the World Resources Institute said Sunday in a talk at the Global Landscape Forum today, forests are “the original decarbonization machines—the carbon capture and storage technique invented by nature.” And I’d add, by far the best invented by anyone—nature or us.

How does this relate to the meanings of “decarbonization,” “climate neutral” and “carbon budget” and the proposed goal of reaching zero greenhouse gas emissions by around 2070? Here, I’m following the definitions in a 2015 paper by Joeri Rogelj and colleagues in the scientific journal Environmental Research Letters, entitled “Zero emission targets as long-term global goals for climate protection.” As they explain:

• Decarbonization means eliminating all CO₂ emissions from the energy/cement sector—i.e. zero CO₂ emissions, but just for that gas and just in those sectors
• Carbon neutrality means zero net CO₂ emissions from all sectors—i.e. that emissions from energy and land are equal to removals (which only can come from the land sector)
• Climate neutrality means zero net emissions of all greenhouse gases from all sectors. Thus, it’s the most complete of the three concepts; it covers the impacts of all the major gases going both into and out of the atmosphere from all human activities. In a common shorthand, it’s the concept that corresponds to “what the atmosphere sees”

 These are the definitions, but why do the differences matter? Here, it’s important to recognize one of the most important discoveries in climate science in the past decade, and perhaps many decades. This is that the increase in the global temperature is essentially proportional to the total amount that has been emitted. Here’s a graph showing this, from the IPCC’s latest assessment report (AR5):

The more we emit, the more temperatures go up. So, how much can we emit if we want temperatures to stabilize – to stop increasing altogether? The answer, clearly, is zero. Or, as one of the papers announcing this discovery was entitled, “Stabilizing climate requires near-zero emissions.”

You may have noted that the graph above shows CO₂ emissions, not all greenhouse gases. This is because eventually, CO₂ is the gas that matters most. That’s because once emitted, it lasts in the atmosphere for many centuries, while methane breaks down over decades. So in the very long term, it’s total CO₂ emissions that need to go to zero, in net terms — i.e., carbon neutrality. But in the sort-of-long term—e.g. the 21st century—all three gases matter, so we will need climate neutrality in order to keep temperatures from continuing to rise throughout our lifetimes.

A very important point is that it’s net emissions that we’re talking about here—emissions minus removals. That’s what adds up to cumulative emissions, which is what determines how much the global temperature rises. So if we want global temperatures to stop rising—i.e., a stable climate—we have to make net emissions equal to zero.

In fact, if we not only want to stabilize temperatures, but stabilize them at a level that avoids dangerous climate change, we actually have to go past zero. That is, we’ve already emitted so much, that we’ll need net negative emissions to get to 1.5 degrees or even probably to 2 degrees of temperature rise.

How is negative emissions possible? Only with the help of that original decarbonization machine, the forest. We’ll need to reduce energy sector emissions by 90 or 95 percent, maintain the current removals from existing forests, and also increase those removals by restoring forests and other natural ecosystems. The new forests’ capacity to remove CO₂ from the atmosphere is quite limited, so they can only do a small part of the job. But it’s a critical part, because it’s what gets us to and past zero net emissions. That is, it’s what makes carbon neutrality and climate neutrality possible. Decarbonization is just not enough.

Now, let’s bring this back to the negotiations and the debate about the collective long-term goal. Sometimes it’s useful to go back to the starting point so as to remember why the world’s nations started this whole process in the first place. In the case of the climate negotiations, that means going back to 1992 and the signing of the United Nations Framework Convention on Climate Change—the “Convention.” And in fact, the Convention says quite clearly, in its Article 2 what is its “ultimate objective.” It’s:

“stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.”

So the Convention is clear – greenhouse gases in the atmosphere have to stabilize. To achieve this, net emissions have to drop to zero. In other words, climate neutrality.

A letter from an eminent group of scientists to the U.S. presidential candidates, just released by UCS,  recognizes this reality. It urges them “to put our nation on a path to a vibrant economy free from carbon pollution by mid-century.” As they indicate, “moving away from fossil fuels…..coupled with increases in carbon uptake in our nation’s forests and soils, can bring us well within reach of an economy free of carbon pollution by mid-century.”

These scientists have recognized that if the world is to achieve climate neutrality, developed countries will need to take the lead. This same recognition is why the Union of Concerned Scientists has adopted, as one of its five Strategic Goals, the achievement of climate-neutrality by the U.S. by the year 2050.

So far, I have focused mostly on the science underlying the choice of a long-term goal. But let me conclude with two other points—one about policy, and one about morality.

The policy point relates to how we go about structuring our approach to zero. Some have argued that we should separate the land sector from the energy sector, with separate accounting for their emissions and removals and different long-term goals. We could do that—but the atmosphere wouldn’t care. It mixes the net emissions all together, irrespective of the sectors from which they came. So it’s the total net emissions from both sectors—all the CO₂ and methane and nitrous oxide that we emit, minus all the CO₂ that we remove, that matters for our climate future.

The moral point goes beyond science. It’s about our responsibility as humans living on earth. If, as Pope Francis put it in Laudato Si’, we “care for our common home,” then we need to realize that “dangerous climate change” doesn’t just mean dangerous for us living humans. The danger we have created is to our children and grandchildren and generations to come, and also to all the marvelous species with whom we share our planet. Climate neutrality is necessary for our future—but for their future too. It means that, as stewards of our common home, we will finally stop breaking it apart.

There’s a new statue standing in front of the Gard du Nord train station here in Paris. It’s called “Angel Bear:”

It’s nothing like any species living on earth today – a bear, but bright red, full of holes, and with enormous wings. But perhaps, at least for this week, it can represent the future that we need to create. We need to begin to fly.