What is human-induced warming?
The safecarbon.org, website shows an up-to-the-second index of human-induced warming relative to the mid-19th century (1861-80) based on the standard “detection and attribution” approach introduced by Hasselmann (1997) and used by the IPCC ever since. This estimates contributions to observed climate change, removing the impact of natural year-to-year fluctuations, by a simple least-squares-fit between observed temperatures and estimated responses to human-induced and natural drivers of climate change. In this case, the responses are provided by the standard simple climate model given in Chapter 8 of IPCC (2013), but the size of these responses (and so the climate sensitivity) is estimated by the fit to the observations (see Box 10.1 of Chapter 10 of IPCC (2013) and here for an explanation).
The human-induced warming index is calculated based on the results of Otto et al (2015), using data and calculations shown in this spreadsheet. The figure below, figure 1a from Otto et al (2015), shows the evolution of the human-induced warming index (orange diamonds) over time. The orange lines show the estimate of human driven temperature change, blue lines the naturally driven (from solar and volcanic change) temperature change and the red lines their combined total. The different lines in each set represent different assumptions about the climate response. The orange diamonds show the best-estimate human-induced warming based on a regression of the observations of global temperature change (black diamonds) onto the human-driven and natural-driven warming. This time series is used for the human-induced warming index in the interactive graphic. The residuals of this regression are shown by the green diamonds, offset in the vertical by 0.5 degrees.
Human-induced warming was 0.909460801 degrees at 00:00:00 on 01/07/2014 and has been increasing at 5.81145e-10 degrees per second ever since.
This figure shows the evolution of the human-induced warming index over time, whereas the interactive graphic on the main page shows the index as a function of cumulative carbon dioxide emissions over the historical period.
What are cumulative carbon dioxide emissions?
Cumulative carbon dioxide emissions is the sum of all emissions emitted since 1870 at any given date. It is the fundamental driver of human-induced warming of the climate system. Cumulative carbon dioxide emissions are calculated from observational data taken from CDIAC and Houghton et al. as in IPCC AR5, with 2012-2014 emissions taken from RCP8.5 emissions calculated at PIK. Cumulative carbon dioxide emissions were 547.54282 GtC at 00:00:00 on 01/01/2015 and have been increasing at 349 tonnes per second since.
Carbon dioxide is a very long-lived gas for which a certain fraction of the emissions will stay in the atmosphere essentially forever. This creates a strong relationship between the total stock of carbon dioxide that enters that atmosphere and the very long-term climate consequences, prompting the focus on cumulative carbon dioxide emissions in climate policy discussions.
Why does this matter?
The latest climate science, summarised in the IPCC 5th Assessment Report, shows a fundamental linear link between human-induced temperature change at any date and the cumulative carbon dioxide emissions before that date. A range of different strengths of this linear relationship are possible, as simulated in the latest generation of global climate models and shown by the range of the pink-coloured plume in the main figure. For any peak-warming target, such as 2 degrees, the pink-plume shows a range of uncertainty over the cumulative carbon dioxide emissions budget to meet the target. The 'We Are Here' marker shows how current human-induced global warming is currently evolving as a function of realised cumulative carbon dioxide emissions.
Due to the uncertainty over the physical climate response to cumulative emissions, shown by the pink-plume, there is a risk that for the best estimate cumulative carbon budget of 1000 GtC for a 2 degreee warming there is the risk of a temperature response up to 3 degrees or as little as 1.5 degrees. Carbon budgets that give 2 degrees of warming could be as much as 1400GtC, or as little as 700GtC. Explicitly linking climate change mitigation policies to a specific carbon budget leads to risks of missing the temperature goal or costly over-mitigation depending on what the carbon budget actually turns out to be. By indexing climate policies to the human-induced warming index instead, this uncertainty over the budget is removed in a climate mitigation context. If policies are designed to evolve with rising human-induced warming such that global emissions are net-zero at the point human-induced warming reaches 2 degrees then the climate goal will be met regardless of the uncertainty in the exact cumulative carbon dioxide budget, allowing climate policies that are "anti-fragile" to uncertainty in the physical climate system.
Warming versus cumulative carbon dioxide emissions
Otto, F.E.L. et al. (2015). Embracing uncertainty in climate change policy. Nature Climate Change.
Hasselmann, K. (1997). Multi-pattern fingerprint method for detection and attribution of climate change. Climate Dynamics, 13(9), 601-611.