The world has an ultimate objective to avoid dangerous interference with the climate system. Prior pages cover some climate system basics and forums to engage for learning, responding and innovating solutions. What's missing? What's still needed? We need to quantify a target and put date on it. As with anything, this can generate focus and energy among diverse individuals and groups to move quickly toward an outcome that is widely seen as desirable and important.
For climate change, world institutions, UNFCCC member states in particular, have indicated strong support for a measurable target (avoid 2°C) and a date (now and forever). On the other side of the coin, the temperature trajectory is powerful and will exceed 2°C if sufficient changes in human activities are not implemented soon enough.
Here, 'Stabilization Step 5' takes a step back to introduce the 'avoid 2°C' target, other temperature targets and some other types of targets. Whether or not 'avoid 2°C' maintains the wide support it has now, it may help to move forward with eyes wide open. Further, multiple targets may be complementary and useful.
A carbon budget quantifies remaining CO2 or greenhouse gas (CO2 equivalent) emissions that may occur before breaching a guardrail target like 'avoid 1.5°C' or 'avoid 450 ppm CO2'. Adopting a cumulative carbon budget reflects a choice to cease emissions before a particular guardrail is reached.
CO2 persists in the atmosphere for periods of time that exceed multiple human lifetimes. Thus, carbon budgets are cumulative over a period of centuries or millenia. They are not reset each year like many financial budgets.
Meinshausen et al. (2009). Greenhouse-gas emission targets for limiting global
warming to 2C. Nature, 458 1158-1163. [.pdf + resources]
Zickfeld, K., Eby, M., Matthews, H. D., Weaver, A. J., & Schellnhuber, H. J. (2009). Setting cumulative emissions targets to reduce the risk of dangerous climate change. Proceedings of the National Academy of Sciences of the United States of America, 106(38), 16129-16134. [abstract + .pdf]
Paris Climate Talks: The 2°C Limit
Nature video examines the science behind the warming target
Source: Nature | November 19, 2015
CO2.earth Global warming update
FiveThirtyEight '16 When will it actually be hotter than 2 degrees?
Carbon Brief '14 Two degrees: Will we avoid dangerous climate change?
Carbon Brief '14 Two degrees: History of the climate change limit
Pathways to 2°C
"There are multiple mitigation pathways that are likely to limit warming to below 2°C relative to pre-industrial levels. These pathways would require substantial emissions reductions over the next few decades and near zero emissions of CO2 and other long-lived greenhouse gases by the end of the century. Implementing such reductions poses substantial technological, economic, social and institutional challenges, which increase with delays in additional mitigation and if key technologies are not available. Limiting warming to lower or higher levels involves similar challenges but on different timescales."
~ IPCC (2014, p. 20)
"UN negotiations on climate change aim to limit warming to 2 °C above pre-industrial temperatures. There is, however, no agreement on how to define pre-industrial temperature."
~ New Scientist, 2015 (attributed to Ed Hawkins, University of Reading, UK)