Understanding climate scenarios

Any given change in the atmosphere toward an increasing global average temperature may be referred to as a climate scenario or warming scenario.

As the climate changes, the ranges and averages that can be expected at a local, regional, and global level change as well. Because the climate system is complex, an increase in global average temperature results in a wide range of changes in heat, precipitation, and dryness. Developing foresight into these wide-ranging changing conditions offers an opportunity to take action to manage the unavoidable and avoid the unmanageable range of futures we now face.

The Probable Futures data and maps offer insight into this range of probable conditions for six warming scenarios: two that are now in the past, one that is probable in the coming years, and three that are possible but currently avoidable. More warming scenarios may be added to the Probable Futures data and maps in the future.

Warming scenarios are typically measured and referred to in increments of global average temperature rise since pre-industrial global average temperature, which for the purpose of climate science and policy is typically considered 1850-1900.

Celsius is typically used as the unit for warming scenarios. Changes in global average temperature of even 0.5°C represent substantially different climates so common increments of warming scenarios include 1°C, 1.5°C, and 2°C. The Probable Futures warming scenarios include 0.5°C, 1°C, 1.5°C, 2°C, 2.5°C, and 3°C. These small-sounding numbers may appear incremental but every increase in global temperature has implications for local and regional weather, natural systems, and society.

When are warming scenarios?

It is common to wonder when warming scenarios have occurred in the past or will occur in the future. Although it is not possible to assign specific dates to specific future or potential warming scenarios without making assumptions about factors such as human behaviors related to emissions, it is possible to predict a range of years when certain warming scenarios may be reached given current rates of emissions.

The following sentences describe when warming scenarios have happened in the past and the range of years when future warming scenarios may be reached in the future.

0.5°C warming scenario

The average surface temperature between 1971 and 2000 was approximately 0.5°C above that of 1850-1900.

1.0°C warming scenario

In 2017 the average surface temperature passed 1.0°C above the 1850-1900 average. Humans have only experienced higher temperatures during a brief period 120,000 years ago. Major biotic changes, including release of greenhouse gasses from thawing permafrost, forest fires, and collapse of Arctic sea ice, have begun contributing to further warming.

1.5°C warming scenario

Reaching 1.5°C is likely by 2030, and the probability of stopping warming below 1.5°C is approaching zero. Standards and expectations based on the past are rapidly becoming outdated. Heat, drought, deluge, and other stresses will increase, and large-scale biotic feedbacks are likely to intensify.

2.0°C warming scenario

The probability of staying below 2.0°C of warming is falling quickly. On the current path of emissions, 2.0°C will likely be passed in the 2040s. However, rapid, dramatic action to get to zero emissions can substantially lengthen the time before reaching 2.0°C, giving society and nature more time to adapt, prepare, and innovate.

2.5°C warming scenario

Rapid, dramatic action to get to zero emissions can substantially reduce the probability of reaching 2.5°C. On the current path, 2.5°C will likely be passed in the 2050s. The earth was last this warm nearly 3 million years ago, when there were no land-based ice sheets other than on Antarctica and Greenland. Stabilizing temperatures at 2.5°C would likely require humans to perpetually offset biotic sources of warming.

3.0°C warming scenario

If we act rapidly to get to zero emissions and find ways to increase carbon stores, we can make reaching 3.0°C a low probability. On the current path of emissions, 3.0°C will likely be passed in the 2060s. At 3.0°C, most regions of the Earth would have entered a different climate, causing severe biological disruptions. The climate is extremely unlikely to be stable at this temperature.