• Injecting reflective nanoparticles/sulfur dioxide into the atmosphere (stratospheric aerosol injection)
• Brightening of low clouds over the ocean by seeding ocean clouds with submicron salt particles
• Using space mirrors, that is, many giant mirrors launched into outer space to reflect sunlight.

SRM is the only option that could cool the planet within years. To be effective at limiting global warming, SRM would need to be maintained for several decades to centuries, depending on the pace of emissions reductions and carbon removal.

• This is an untested planetary intervention
• There could be disparate effects on certain regions

Major volcanic eruptions, which introduce large amounts of sulphate particles into the stratosphere, provide a natural analogue for SRM deployment (Figure 4). For example, the 1991 Mount Pinatubo eruption caused global annual-mean cooling of about 0.3–0.5°C in the following two years. An SAI deployment would inject aerosols continuously into the stratosphere. It is estimated that continuous injection rates of 8–16 Tg of sulphur dioxide (SO2) per year (approximately equivalent to the estimated injection amount of Mount Pinatubo in the single year of 1991) would reduce global mean temperature by 1°C. An operational SAI deployment could be scaled up to produce global cooling of 2–5°C, albeit with diminishing returns at higher rates of injections.

In Russia, 1601-1603 brought the worst famine in the country’s history, leading to the overthrow of the reigning tsar. Records from Switzerland, Latvia and Estonia record exceptionally cold winters in 1600-1602; in France, the 1601 wine harvest was late, and wine production collapsed in Germany and colonial Peru. In China, peach trees bloomed late, and Lake Suwa in Japan had one of its earliest freezing dates in 500 years.