The clock is ticking on our planet's future. With every passing year, the window to prevent catastrophic climate change narrows, making it crucial for us to understand and act on the nuances of greenhouse gas emissions. According to the Intergovernmental Panel on Climate Change (IPCC), the world must act, dramatically and quickly, to avert the worst consequences of global climate change resulting from greenhouse gas emissions.
As governments and corporations around the world set climate goals and take action towards them, a pervasive challenge arises around metrics for understanding the impact of different greenhouse gas emissions. There are many contributors to climate change; carbon dioxide (CO2) is the most prevalent greenhouse gas, followed by methane (CH4), nitrous oxide (N20), and fluorinated gases. Effective action then requires an understanding of each of these gas’s ability to trap heat in the Earth’s atmosphere and contribute to climate change.
For example, short lived climate pollutants (SLCPs) are a subcategory of greenhouse gases that remain in the atmosphere for less time than carbon dioxide but have a potent impact on near-term warming. The concept of Global Warming Potential (GWP) has become a popular measuring tool to understand the impact of these pollutants on the climate. GWP is a relative measure using CO2 as the reference gas to describe how much impact a greenhouse gas will have on atmospheric warming over a certain time period. GWP plays a key role in climate policy and in goalsetting, as it allows stakeholders to compare the emissions of different greenhouse gases and prioritize actions accordingly.
The most commonly used metric is GWP100, which describes how much a certain gas contributes to climate change over a period of 100 years. GWP20 does the same for a period of 20 years. To illustrate, let’s look at methane. Methane is non-cumulative, as opposed to a stock gas like CO2, and has a short atmospheric lifespan of around 12 years The GWP100 and GWP20 of methane, as determined by the most recent IPCC report (AR6), is 27-30 and 81-83 CO2e respectively. To address these disparities, academics developed a new metric, GWP*, to take into account the lifetimes and temperature responses of different greenhouse gases so that the results align with modeled warming impacts of emissions scenarios.
While GWP100 and GWP20 provide useful benchmarks, they don't tell the whole story. This is where GWP* comes in, offering a more dynamic way to measure the warming impact of greenhouse gases over time. GWP* includes feedback of removals and compound long term warming, which more accurately describes the resulting temperature impact from GHG emissions. While GWP20 or GWP100 calculates fixed time metrics with average values, GWP* calculates the warming potential at a given point in time. Consequently, there is no constant GWP* value for methane. GWP* calculates CO2 warming equivalent (𝐸CO2we) as opposed to the emission based calculation of CO2 equivalent. See Figure 1 for graphs demonstrating the disparities when calculating with GWP100 vs. GWP* for various CH4 emission scenarios.
Figure 1: GWP* and GWP100 in various methane scenarios: A) constant emissions, B) linearly decreasing emissions rates, and C) a rate of decline where the methane emissions are equivalent to 0 CO2we. The red line shows annual CO2e using GWP100, and the blue line shows annual CO2we using GWP*. The orange dashed line shows the relative temperature change from the methane emissions.
The drastically different values that methane presents, depending on the metric used, elucidate the need for accurate description of climate impacts of emissions, and highlight the consequences of different methodological choices on goal setting and progress tracking.
Forward thinking academics are continuing to explore this topic, with a recent paper advising to move away from a single-basket approach entirely. As we push towards a sustainable future, understanding the true impact of our emissions is more crucial than ever. By refining our metrics and embracing new approaches, we can better target our efforts to safeguard our planet for future generations.