Editing IPCC:AR6/WGI/Chapter-2 (section)

==== 2.3.4.1 Seasonal Cycle of Atmospheric CO <sub>2</sub> ====

<div id="h3-27-siblings" class="h3-siblings"></div>

The AR5 noted that because CO <sub>2</sub> uptake by photosynthesis occurs only during the growing season, the greater land mass in the NH imparts a characteristic ‘sawtooth’ seasonal cycle in atmospheric CO <sub>2</sub> . The SRCCL similarly stated that due to strong seasonal patterns of growth, NH terrestrial ecosystems are largely responsible for the seasonal variations in global atmospheric CO <sub>2</sub> concentrations. Neither AR5 nor SRCCL made a confidence statement about observed changes in the amplitude of the seasonal cycle of CO <sub>2</sub> .

In situ observations of CO <sub>2</sub> generally depict a rising amplitude of the seasonal cycle over the past half century, especially north of about 45°N (Figure 2.30). For example, an amplitude increase of 6 ± 2.6% per decade has been observed at the Barrow surface observatory in Alaska over 1961–2011 ( [[#Graven--2013|Graven et al., 2013]] ), with slightly slower increases thereafter. Aircraft data north of 45°N exhibit an amplitude increase of 57 ± 7% at 500 mb versus an increase of 26 ± 18% for 35°N–45°N between field campaigns in 1958–1961 and 2009–2011 ( [[#Graven--2013|Graven et al., 2013]] ). Increases in amplitude for the period 1980–2012 are apparent at eight surface observatories north of 50°N ( [[#Piao--2018|Piao et al., 2018]] ), related primarily to a larger drawdown in June and July. Trends in seasonal cycle amplitude at lower latitudes are smaller (if present at all); for instance, the increase at the Mauna Loa observatory in Hawaii since the early 1960s is only about half as large as at Barrow ( [[#Graven--2013|Graven et al., 2013]] ), and only one other low-latitude observatory has a significant increase from 1980–2012 ( [[#Piao--2018|Piao et al., 2018]] ). There is a weak signal of an increase in amplitude at the Sinhagad observatory in western India in recent years ( [[#Chakraborty--2020|Chakraborty et al., 2020]] ). Generally speaking, larger increases in the Arctic and boreal regions are indicative of changes in vegetation and carbon cycle dynamics in northern ecosystems ( [[#Forkel--2016|Forkel et al., 2016]] ), though increased carbon uptake can also result from other factors such as warmer- and wetter-than-normal conditions.

<div id="_idContainer075" class="Basic-Text-Frame"></div>

[[File:be5dbe4f2d92ae62b5a85f9cd87ba02b IPCC_AR6_WGI_Figure_2_30.png]]

'''Figure 2.3''' '''0 |''' '''Changes in the amplitude of the seasonal cycle of CO''' <sub>2</sub> '''. (a)''' Observed peak-to-trough seasonal amplitude given by the day of year of downward zero crossing, of CO <sub>2</sub> concentration at Barrow (71°N, blue) and Mauna Loa (20°N, black). Seasonal CO <sub>2</sub> cycles observed at '''(b)''' Barrow and '''(c)''' Mauna Loa for the 1961–1963 or 1958–1963 and 2017–2019 time periods. The first six months of the year are repeated. Reprinted with permission from AAAS. Further details on data sources and processing are available in the chapter data table (Table 2.SM.1).

Recent satellite-based, global-scale estimates of seasonal variations in atmospheric CO <sub>2</sub> for the period 2003–2018 show that the seasonal variations in the SH are out of phase with those in the NH ( [[#Reuter--2020|Reuter et al., 2020]] ), which is consistent with the phenological shifts in primary productivity between hemispheres. The net effect of the phase shift between the two hemispheres is to dampen the amplitude of the global average seasonal cycle. These integrated results also show that the amplitude of the oscillations has been increasing in the SH, from about 2009, but comparison with data from Baring head suggests that periods of high seasonal oscillation had occurred at that location in the SH prior to 1995.

In summary, there is ''very'' ''high confidence'' that the amplitude of the seasonal cycle of atmospheric CO <sub>2</sub> has increased at mid-to-high NH latitudes since the early 1960s. The observed increase is generally consistent with greater greening during the growing season and an increase in the length of the growing season over the high northern latitudes. Similarly, globally-integrated results from the SH also show an increase in seasonal amplitude of atmospheric CO <sub>2</sub> signal, from around 2009 to 2018 ( ''low confidence'' ).

<div id="2.3.4.2" class="h3-container"></div>

<span id="marine-biosphere"></span>