land clearance has appreciably altered compared with earlier years. The cause of
this decrease in the rate of change appears to be small alterations in the natural
fluxes between the atmosphere and land surfaces and the oceans. The large two-
way fluxes between these latter reservoirs and the atmosphere mean that only a
small imbalance between the up and down fluxes is enough to lead to an observ-
able change in the atmospheric CO 2 concentration. The reasons for such imbal-
ances are largely unknown at present, although possible relationships with, for
example, El Niño events (as indicated in Fig. 7.8) are the subject of considerable
research effort. El Niño events occur every few years in the tropical Pacific ocean
when abnormally warm water near the surface in the eastern part leads to alter-
ations in water circulation and heat exchange with the atmosphere. These cause
disruption to fisheries along the coast of Ecuador and Peru, and the altered
atmospheric circulation leads to changes in climate throughout the Pacific region
and in many other parts of the world.
On the timescale of thousands of years it is clear that changes in the land and
ocean reservoirs have led to imbalances in their CO 2 fluxes with the atmosphere.
The best evidence for this comes from ice cores and the record of atmospheric
composition preserved in them. Figure 7.10 shows how atmospheric CO 2 con-
centrations and earth-surface temperatures have changed over the last 420 000
years, as recorded in the Vostok ice core from Antarctica. There have clearly been
dramatic changes in atmospheric CO 2 levels over this period and the most likely
explanation for these shifts is that they arise from (temporary) imbalances
between the inter-reservoir fluxes.
While examining Fig. 7.10, it is worth noting that the excursion in atmos-
pheric CO 2 over this 420 000-year period (about 110 ppm) is only marginally
greater than that achieved by human activities over the last 200 years (90 ppm),
as shown in Figs 7.1 and 7.2. A second point to note from Fig. 7.10 is the close254 Chapter SevenCO
(ppm) 2
Temperature (°C)180220260300340380–10–8–6–4–2024Age (years ¥ 103 BP)400 350Temperature300 250 200 150 100 50 0CO 2Fig. 7.10CO 2 concentration in the atmosphere and estimated temperature changes during the past 420 000
years, as determined from the Vostock ice core from Antarctica. BP, before present. Data from Petit et al.
(1999).