When introducing climate engineering, I often begin with something deceptively simple: the napkin diagram. The story is regarding an esteemed colleague John Sheppard, who sketched the original while talking through climate pathways over a meal.
The power of the diagram lies in its clarity. Time runs along the horizontal axis, temperature on the vertical. Stretch the timescale across a few centuries and a stark picture emerges of where we are headed, and what must be done to avoid dangerously high levels of warming.
“We do not have the luxury of pretending one lever will save us. The diagram is blunt, but it is also constructive. It shows a path that can keep global temperature within manageable bounds if we act decisively and in concert.”
Professor Hugh Hunt
What the napkin diagram shows:
Start with business as usual. If we continue to burn fossil fuels and allow greenhouse gases to accumulate, temperatures rise. The line climbs steadily. There is nothing speculative about this trajectory; it follows directly from the physics of greenhouse gases and the pace at which we are still adding them to the atmosphere.
Now add what should happen under the UNFCCC and the COP process: emissions reduction. This is step one. As countries implement policies to cut emissions, the temperature curve should begin to bend down. The trouble is the bend is far too gentle and too late. The turnaround is not happening quickly enough to keep us within safe limits.
Step two is carbon dioxide removal (CDR). Even with strong emissions cuts, we need to draw down CO₂ already in the air. That can involve nature-based approaches like planting trees or restoring ecosystems, enhancing ocean uptake, or using engineered methods such as direct air capture. All are essential. Yet scaling them to meaningful levels will take years, more likely decades.
The napkin diagram makes the implication inescapable: between where we are now and where we need to be lies a “bump” or overshoot. During that period, global temperature could exceed 2°C, possibly 3°C, above pre-industrial levels. That is a realm of escalating risk for ecosystems, food and water security, and the stability of our climate system.
Why solar radiation management enters the frame:
This is where step three comes in: solar radiation management (SRM). Sometimes called “peak shaving,” SRM is not a cure for climate change. It does not remove CO₂. It buys time by reflecting a small fraction of sunlight back to space, reducing temperatures while emissions fall and CDR scales up.
Two approaches illustrate the idea. Marine cloud brightening aims to increase the reflectivity of low marine clouds. Stratospheric aerosol injection (SAI) seeks to mimic the cooling effect observed after large volcanic eruptions by dispersing reflective particles high in the stratosphere.
On the napkin diagram, SRM is the shaded area that trims the peak of warming during the overshoot years. It is a temporary, targeted intervention to keep temperatures at manageable levels, not a replacement for emissions cuts or carbon removal. All three steps are required.
I am often asked how long SRM would be needed. The honest answer is: some decades, depending on how rapidly we decarbonise and deploy CDR at scale. The faster we move on steps one and two, the shorter and smaller the shaded SRM area becomes. The slower we move, the larger and longer it must be to keep temperatures in check. That is the uncomfortable arithmetic the napkin diagram forces us to confront.
Why this matters for COP:
The UNFCCC COP meetings exist to accelerate step one and to bring step two from promise to reality. The challenge is that the pace of action has not yet matched the pace of warming. The napkin diagram should be on the table—literally—when negotiators meet. It communicates three critical points.
First, emissions reduction remains the non-negotiable foundation. Cut faster, bend the curve sooner, reduce the overshoot. Every fraction of a degree avoided reduces risk.
Second, CDR is not optional. Even with ambitious mitigation, removal will be required to bring atmospheric concentrations down. That means serious strategies for research, standards, finance, and deployment, while safeguarding ecosystems and communities.
Third, and most contentious, SRM needs governance, transparency and research now, not later. The question is not whether SRM is preferable to mitigation or removal—it is not—but whether it may be necessary to reduce the duration and severity of overshoot while the other two do their work. Ignoring that possibility does not make the risk disappear. Good governance, international oversight, and rigorous science are the minimum conditions for any consideration of SRM, especially SAI, with its global reach and ethical implications.
A practical framework for what comes next
If the napkin diagram has a lesson, it is urgency with sequencing. Accelerate emissions reduction. Build durable capacity for CDR. Establish robust international frameworks to evaluate SRM, including strict guardrails, monitoring, and pathways for redress. Keep all three strands coordinated so that SRM, if used, is temporary and strictly subordinated to cutting emissions and removing carbon.
The beauty of the napkin sketch is that it strips away euphemism. The longer the delay on steps one and two, the larger the overshoot and the stronger the case for a carefully governed SRM bridge. The shorter the delay, the less SRM is needed and the sooner we can retire it. That is a choice, not an inevitability.
We do not have the luxury of pretending one lever will save us. The diagram is blunt, but it is also constructive. It shows a path that can keep global temperature within manageable bounds if we act decisively and in concert. That is the task before COP: to turn the napkin into a plan, and the plan into action, before the bump becomes the cliff.
