We project scientifically accurate information on the top of the south face of the Green Building. The building is both the tallest building in Cambridge, and home to the MIT Department of Earth, Atmosphere and Planetary Sciences. The MIT Climate Clock features four lines that highlight the need for climate action.

Goal

to limit global temperature increase to 1.5°C (2.7°F) which is the target embraced in the COP21 Paris Agreement. The Paris Agreement, signed in 2015, is a legally binding international treaty on climate change to "limit global warming to well below 2, preferably to 1.5 degrees Celsius, compared to pre-industrial levels"

Carbon Dioxide Emissions Budget

a very large number from which one derives the actual time of the climate clock. The "Carbon Dioxide Emissions Budget" represents how much carbon dioxide the world as a whole can release into the atmosphere with at least a 66% chance of avoiding global average temperature increases of 1.5°C (2.7°F).

Deadline

Credible current projections suggest we have about seven years left at current emission rates. The "Deadline" is another way of expressing the carbon dioxide emissions budget. It's the time we have left to take decisive action to limit global average temperature increase to 1.5°C (2.7°F).

Lifeline

The "Lifeline" represents the percentage of the world's energy that is currently provided through renewable sources.

Our carbon dioxide budget data is aligned with the latest IPCC AR6 2021 report (see table SPM.2). The resulting deadline should closely mirror the numbers displayed on the Mercator Institute website . Lifeline estimates are the result of counting forward from the middle of 2020 starting from 12.55%, based on data from the World Energy 2021 Special Report.

In order to figure out the time available for action, one can compute a carbon dioxide budget. The budget answers the question "how much carbon dioxide can we put into the atmosphere, globally, before reaching 1.5°C above pre-industrial levels?"

The Account Limit: Carbon Dioxide Threshold

Using measurements and models, scientists calculated the maximum amount of carbon dioxide that can be in the atmosphere without exceeding a certain average temperature above pre-industrial levels by 2100. The number is approximately 400 giga-tonnes of carbon dioxide [IPCC AR6].

The Expenses: Atmospheric Carbon Dioxide

But how much carbon dioxide is already in the atmosphere? Measurements are taken regularly at Mauna Loa Observatory in Hawaii. At the start of 2022, there was already 417 ppm of carbon dioxide in the atmosphere, up from under 300ppm in 1950 [IPCC AR6, A.1.1]. Every year that more carbon dioxide is emitted into the atmosphere, atmospheric carbon dioxide "expenses" increase. By the same token, as we reduce our CO₂ emissions, or sequester carbon, our expenses decrease. If our sequestration equals our emissions, we've hit "net-zero," such that we're removing as much carbon from the atmosphere as we're putting into it.

The Balance: CO₂ Emissions Budget

The balance is simply the limit minus the carbon dioxide already in the atmosphere. We call this number the "CO₂ Emissions Budget." It decreases each day as more carbon dioxide goes into the atmosphere.

For more resources describing carbon budgets, see Climate Change: What is the carbon budget? and The Carbon Budget - what is it and why is it important?

Arriving at a CO₂ Budget

To convert the carbon dioxide budget into a countdown clock, we make assumptions about our world. We assume that global carbon dioxide emission rates stay fixed, and calculate how much time is left before the budget is used up. We use the assumption that the CO₂ Emissions budget is reduced by 42.2 Gt of CO₂ per year, or 1,337 tonnes/sec based on this reference: Mercator Research Institute on Global Commons and Climate Change (MCC). The Mercator Research Institute relies on data from the recent IPCC Special Report on Global Warming of 1.5°C . Calculations are based on the year 2020, counting down the expended carbon budget from there. The budget specifically addresses carbon dioxide, not directly including additional greenhouse gasses such as methane. Carbon dioxide is particularly important, as it remains in the atmosphere on the order of 300-1000 years.

There are other limitations to our budget calculation. The annual emissions of 42.2 Gt-CO₂/yr from the MCC include sources like "burning fossil fuels, industrial processes and land-use change". They do not factor in additional emissions from long-term changes in natural sources and sinks due to potential thresholds and tipping points, such as such as permafrost melting causing the release of gas hydrates, melting of the Greenland or Antarctic ice sheets, slowing down the North Atlantic deep ice circulation or the dieback of the Amazon rainforest. While the effects of these natural sources are large, the timescale for these changes may be on the order of decades to centuries, which is much longer than the timescale of our climate clock. Thus we chose not to include these in the budget (but will continue to revisit those issues).

COVID-19 temporarily changed our global carbon dioxide emission rate. According to the Global Carbon Project report from December 11, 2020, "Total CO₂ emissions from human activities (from fossil CO₂ and land-use change) are set to be around 40 billion tonnes of CO₂ in 2020, compared to 43 billion tonnes of CO₂ in 2019." Furthermore, "global fossil CO2 emissions are expected to decline approximately 2.4 billion tonnes of CO2 in 2020 (-7%), a record drop. The decrease in emissions, caused by COVID-19 confinement measures in place, brings global fossil CO₂ emissions to 34 billion tonnes of CO₂... Emissions in 2019 were only 0.1% above emissions in 2018, at 36.4 billion tonnes of CO₂."

Since 2020, we have begun to see a rebound effect, however. Thus we can approximate total annual emissions, mostly from fossil fuels and land use changes, as 42.2 GtCO2 for the purposes of our climate clock.

Of course there is uncertainty in our future emissions. Our goal is not to predict the future, but rather to ignite action to move towards a liveable future for humanity. Visit the Global Carbon Project to learn from scientists about how international responses to COVID-19 have impacted emissions, and thus the carbon dioxide budget.

Measuring the Progress of Renewable Energy

Energy produced from burning fossil fuels contributes more than any other source to the increasing levels of carbon dioxide in the atmosphere. Sourcing energy from renewables instead means less carbon going into the atmosphere. It was estimated in the World Energy Special Report 2021 by BP that in 2020, 12.55% of the world's energy came from renewable sources and hydro-electric: 5.7% from renewables, and 6.86% from hydro [World Energy Special 2021 Report, page 11]. Including nuclear leads to ~16.8%. Energy consumption consists of electricity, transport and heating energy. When examining just electricity, renewables (including hydro) comprise 27.8% of the electricity sources in the world [WE report, page 65]. In our accounting, renewables include solar, wind, hydropower, and modern bio-fuels. We do not include nuclear, or traditional biofuels (such as fuel-wood, forestry products, animal and agricultural waste) in the estimate. Around the world, even as the supply of energy increases, more and more energy is derived from renewable sources.

To pass net-zero, the point when globally we begin extracting carbon from the atmosphere, we need this percent to increase as much as possible towards 100%. According to World Energy data, the growth rate of renewables (and biofuels, no hydro) for 2020 was 12.5%. In fact, despite decreases in energy consumption, renewable generation still increased. For hydro it was 1% with 4296 Terra-watt-hours of energy from hydro.

For our projection, we begin at 12.541734% at the middle of 2020, and approximate the increase of renewables based on 3.501617*10^-8 %/sec, which comes from (12.541734%-11.43746%)/(365*24*3600s). If nuclear were included as a low-carbon source, the lifeline would be roughly 4.24% higher.