finding 4.5 : key-finding-4-5

The observed increase in global carbon emissions over the past 15–20 years has been consistent with higher scenarios (very high confidence). In 2014 and 2015, emission growth rates slowed as economic growth has become less carbon-intensive (medium confidence). Even if this trend continues, however, it is not yet at a rate that would limit the increase in the global average temperature to well below 3.6°F (2°C) above preindustrial levels (high confidence).

This finding is from chapter 4 of Climate Science Special Report: The Fourth National Climate Assessment: Volume I.

Process for developing key messages: The key finding is based on basic physics relating emissions to concentrations, radiative forcing, and resulting change in global mean temperature, as well as on IEA data on national emissions as reported in the peer-reviewed literature.

Description of evidence base: Observed emissions for 2014 and 2015 and estimated emissions for 2016 suggest a decrease in the growth rate and possibly even emissions of carbon; this shift is attributed primarily to decreased coal use in China although with significant uncertainty as noted in the references in the text. This statement is based on Tans and Keeling 2017;4520c606-c217-4ad8-a152-7afba519d418 Raupach et al. 2007;32311a94-ea1d-41f1-9c85-9e34f9a648cf Le Quéré et al. 2009;5972d180-5f4d-4bdb-ac46-b7e0308320a4 Jackson et al. 2016;90d7846d-254b-44b3-95c4-c0d61101fccc Korsbakken et al. 20165ee463fc-ba27-449d-bd77-2417f252e6da and personal communication with Le Quéré (2017).

The statement that the growth rate of carbon dioxide increased over the past 15–20 years is based on the data available here:

The evidence that actual emission rates track or exceed the higher scenario (RCP8.5) is as follows. The actual emission of CO2 from fossil fuel consumption and concrete manufacture over the period 2005–2014 is 90.11 Pg.60d2b9e1-2b2a-4cce-8bc3-9f91ec68364f The emissions consistent with RCP8.5 over the same period assuming linear trends between years 2000, 2005, 2010, and 2020 in the specification is 99.24 Pg.

Actual emissions: and Le Quéré et al.60d2b9e1-2b2a-4cce-8bc3-9f91ec68364f

Emissions consistent with RCP8.5

The numbers for fossil fuel and industrial emissions (RCP) compared to fossil fuel and cement emissions (observed) in units of GtC are

   RCP8.5    Actual    Difference
2005       7.97       8.23       0.26
2006       8.16       8.53       0.36
2007       8.35       8.78       0.42
2008       8.54       8.96       0.42
2009       8.74       8.87       0.14
2010       8.93       9.21       0.28
2011       9.19       9.54       0.36
2012       9.45       9.69       0.24
2013       9.71       9.82       0.11
2014       9.97       9.89      -0.08
2015      10.23       9.90      -0.34
Total      99.24    101.41       2.18

New information and remaining uncertainties: None

Assessment of confidence based on evidence: Very high confidence in increasing emissions over the last 20 years and high confidence in the fact that recent emission trends will not be sufficient to avoid 3.6°F (2°C). Medium confidence in recent findings that the growth rate is slowing. Climate change scales with the amount of anthropogenic greenhouse gas in the atmosphere. If emissions exceed those consistent with RCP8.5, the likely range of changes in temperatures and climate variables will be larger than projected.

This finding was derived from figure -.2: Confidence / Likelihood

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