Climate science cannot tell us what to value. It can tell us what our choices will do. This final lesson brings the course together: how much more carbon the atmosphere can take before key thresholds are passed, what it would take to stop warming, and how a planet-wide problem connects to the dignity of each person living on it. Here we stay close to the physical evidence rather than the moral debates.
Cumulative carbon
Because CO2 lingers for centuries, warming depends mainly on the total amount emitted over time, not the rate in any one year. Each additional 1,000 billion tonnes of CO2 warms the planet by roughly 0.45°C. This simple, nearly straight-line relationship is one of the most useful findings in climate science. It leads to the idea of a carbon budget: the total CO2 that can still be released while keeping warming below a chosen level.
How much is left
Human activity emitted roughly 40 billion tonnes of CO2 in 2024, most of it from fossil fuels and the rest from deforestation and other land use. Scientific updates published in 2025 estimated that the remaining budget for a 50 percent chance of holding warming to 1.5°C was only about 130 billion tonnes from the start of that year, roughly three years of current emissions. The budget for 2°C is several times larger, but still small compared with the fossil fuels in known reserves.
Net zero stops the rise
A hopeful finding comes from the same physics. Models indicate that once CO2 emissions reach net zero, meaning any remaining emissions are balanced by removals, global temperature roughly stops climbing within a decade or two. The warming already caused will not reverse on its own for a very long time, but further warming is not locked in. To keep warming near 1.5°C, global CO2 emissions would need to reach net zero around 2050; for 2°C, around the 2070s.
Where we are heading
In the 2015 Paris Agreement, nearly every nation agreed to hold warming "well below" 2°C and to pursue efforts to limit it to 1.5°C. Pledges and policies since then have bent the curve. Before Paris, analysts projected warming of around 4°C by 2100 under existing policies. Recent estimates for current policies are in the range of about 2.5 to 3°C, still well above the goal.
The tools are known. Electricity from solar panels and wind turbines has become among the cheapest ever built, with solar module prices falling by around 90 percent during the 2010s. Electric vehicles, heat pumps, efficiency, protecting forests and plugging methane leaks all cut emissions. Every region will also need adaptation, such as heat warning systems, stronger coastal defenses and drought-resistant crops, because some change is already here.
When science meets misinformation
A 2023 study in Science examined projections made by Exxon's own scientists between 1977 and 2003. Their projections of global warming were about as accurate as those of independent academic and government scientists. Yet for years parts of the industry funded public campaigns emphasizing doubt. Knowing the evidence in this course is the best defense against confusion: the physics is old, the measurements are many, and the fingerprints point to the cause.
Worked example: doing the budget arithmetic
Use the course's numbers to see why timing matters.
- Start with the warming per tonne: about 0.45°C per 1,000 billion tonnes of CO2.
- Apply current emissions: 40 billion tonnes a year for 25 years is 1,000 billion tonnes, adding roughly another 0.45°C on top of today's warming.
- Try a steady decline: if emissions fall in a straight line from 40 billion tonnes to zero over 25 years, the total is half as much, about 500 billion tonnes, adding about 0.23°C.
- Compare: the path, not just the endpoint, decides the final temperature. Earlier cuts mean less warming for everyone.
Our place on one small world
Climate science is, in the end, a story of human curiosity and cooperation: Foote's cylinders, Keeling's patient decades on a volcano, thousands of floats drifting through dark oceans, and scientists in every country checking one another's work. It reveals a shared atmosphere that ignores every border. Universal Humanism holds that each person has equal dignity and that we are responsible to one another, including to people not yet born. The evidence gives that idea a concrete shape. The air we change is the air everyone breathes, and every tenth of a degree we avoid is measured in lives, homes and livelihoods. Understanding the science clearly, and passing it on honestly, is one of the ways we take care of our only home and one another.