1) Continental United States
The NOAA Climate Monitoring and Diagnostics Laboratory (CMDL), in cooperation
with several other organizations, operates a network of 16 Dobson ozone spectrophotometers
spread across the continental U.S., as well as at Barrow, Alaska, Mauna Loa, Hawaii, and
American Samoa. These instruments are part of a global network of approximately 60 such
instruments that are calibrated against the World Standard Dobson Spectrophotometer
maintained by NOAA/CMDL. At four stations spanning the middle of the United States between
37° and 40°N (Fresno, CA; Boulder, CO; Nashville, TN; and Wallops Island, VA), total
column ozone amounts during 1999 (red curve) remained well below pre-1980 levels (Fig. 65). The low 1999 ozone amounts are comparable to those
observed during most of the 1990s, but contrast with the relatively high amounts observed
in some years in association with the quasi-biennial oscillation (QBO). Overall, average
total column ozone has declined by about 5% over the past two decades (blue trend line) at
these stations, with the rate of decline slowing in the past five years.
At the Mauna Loa Observatory (20°N) in Hawaii, total column ozone
concentrations exhibit a strong annual cycle (Fig. 66), although
there is also some variability related to the QBO and to the long-term trend. Prior to
1977, there was no discernible trend in ozone amounts at this site. However, ozone has
decreased at a rate of 0.6% per decade since that time, which is notably less than the
average ozone decrease of 2.5% per decade observed at the midlatitude U. S. sites.
2) Southern Hemisphere Ozone
Total column ozone data were obtained from the National Aeronautics and
Space Administration Solar Backscatter Ultraviolet Spectrometer (SBUV) instrument on
Nimbus-7 (1979 through 1988), and the SBUV/2 instruments on NOAA-11 (January 1989 through
August 1994), NOAA-9 (September 1994 through June 1997), and NOAA-14 (beginning in July
1997). Data from the SBUV instruments are only available during daylight viewing
conditions, which results in an absence of data over the polar latitudes during winter.
Other sources of ozone data include Dobson spectrophotometer readings and measurements
from balloon-borne ozonesondes, both of which are obtained from the NOAA CMDL.
The ozone hole, denoted by total column ozone concentrations less than
220 Dobson units (DU), typically reaches its peak areal extent in early October. During
the last 20 years the average size of the ozone hole (Fig. 67)
has increased from 1. 5 x 106 km2 in 1982 to the record value of
16.7 x 106 km2 in 1998. During 1999, the average size of the ozone
hole decreased to 16.2 x 106 km2, slightly less than the 1998 value.
Daily measurements indicate that the maximum areal extent of the ozone
hole during 1999 was 22 x106 km2 (Fig. 68a),
which is less than the record value of more than 25 x 106 km2
observed during 1998. In recent years the ozone hole has also exhibited a tendency to last
later into the year, as was observed during NovemberDecember 1999 when several daily
records of maximum areal extent were established. The ozonesonde data also indicate that
29 September 1999 was the earliest date for the minimum total ozone of the season. In
previous years, the minimum ozone has always occurred between October 415. Thus, the
overall duration of the ozone hole was longer during 1999 than has been observed in past
Total ozone concentrations are closely coupled to lower stratospheric
temperatures through photochemistry. Extremely low stratospheric temperatures (below
80°C) contribute to the formation of polar stratospheric clouds (PSCs), which
enhance the production and lifetime of reactive chlorine, thereby leading to ozone
depletion (WMO/UNEP 1994). The areal extent of the ozone hole is also closely related to
the polar stratospheric vortex, which isolates and concentrates the chemicals that destroy
ozone at low temperatures. From early October through early December the areal extent of
this vortex was larger than the long-term average (Fig. 68b), but
considerably smaller than the record values observed during 1998.
The evolution of conditions accompanying the 1999 ozone hole was
obtained from 68 ozonesondes flown at the South Pole (Fig. 69a).
Total ozone for each flight is shown (red curve) along with the average temperature in the
2024 km layer (black curve). Following sunset on March 20, stratospheric
temperatures began dropping steadily from 50°C as the polar vortex developed over
the Antarctic continent and began to isolate the stratospheric air from warmer midlatitude
intrusions. By mid-June, temperatures in the 2024 km layer had dropped to below
90°C, which is well below the threshold for polar stratospheric cloud formation,
and subsequently remained at this level until early September. In fact, a near record
minimum temperature of -94.7°C (178.5 K) was measured at 21.5 km on 6 August 1999. At
these temperatures the chlorine activation reactions occur on the PSC surfaces, producing
easily-photolyzed forms of chlorine compounds.
At sunrise on September 23 the free chlorine was released, which
initiated the catalytic reactions that destroyed ozone. Severe ozone depletion was then
evident from late September into early December 1999. The primary breakup of the ozone
hole then occurred in early December (Fig. 68a) when the overall
stratospheric polar vortex began to dissipate and air of midlatitude origin moved into the
Ozone concentration profiles from three ozonesondes during 1999 show
the pre-ozone hole conditions on 28 July (Fig. 69b), minimum
total ozone conditions on 29 September (Fig. 69c), and post-ozone
hole conditions on 23 December (Fig. 69d). The pre-ozone profile
indicates a maximum in ozone between 1520 km, which is the primary contributor to
the total column ozone of 255 DU. In contrast, the 29 September profile shows essentially
complete ozone depletion in this layer, with total ozone of 90 DU on that day approaching
the record low value of 88 DU observed in 1993. After ozone recovery, the maximum in ozone
became established between 1823 km (Fig. 69d). Thus, it is
evident that the ozone hole reflects the annihilation of the primary ozone maximum that
normally occurs between 1523 km. It is also evident that stratospheric chlorine
levels still remain at a "saturation" level in the main ozone depletion region
above the South Pole.
Initial signs of Antarctic ozone recovery are expected within the next
10 years (Hofmann et al., 1997) when stratospheric chlorine concentrations begin returning
to pre-1980 levels. This recovery should first appear as a decrease in both the ozone loss
rate and in the extent of the ozone depletion near the top of the main depletion layer.
b. Carbon Cycle Greenhouse Gases
The three major carbon cycle gases are carbon dioxide (CO2,
methane (CH4), and carbon monoxide (CO), with each having its own impact on
climate. Methane and CO2 have direct effects on climate due to their
significant absorption of terrestrial IR radiation. While CO does not absorb terrestrial
IR strongly, it and CH4 indirectly affect climate through their atmospheric
chemistry. Both species are removed from the atmosphere predominantly by chemical reaction
with hydroxyl radical (OH). The OH concentration depends on the amounts of CH4,
CO, NOx, O3, and nonmethane hydrocarbons in the atmosphere. Changes
in the atmospheric burdens and distributions of these compounds can affect OH
concentrations and, therefore, the residence times of CH4 and other greenhouse
gases which are treated under the Kyoto protocol [such as the hyrdofluorocarbon compounds
(HFCs)]. Oxidations of CH4 and CO also affect the concentration and
distribution of tropospheric ozone, another strong greenhouse gas.
1) Carbon Dioxide
After water vapor, CO2 is the most important infrared
absorbing (i.e., greenhouse) gas in the atmosphere. Since the late 1800s atmospheric CO2
levels have increased approximately 30%, due primarily to emissions from combustion of
fossil fuels and to a lesser extent from deforestation (Keeling et al. 1995). The total
sink and the partitioning into marine and terrestrial components varies significantly from
year to year (Conway et al.1994; Ciais et al. 1995). A better understanding of the
processes that remove CO2 from the atmosphere and how these processes respond
to climate fluctuations will enable better predictions of future CO2 levels,
which will in turn decrease the uncertainty associated with models of future climate.
Current modeling approaches to understanding the carbon budget depend
on atmospheric measurement data either as a constraint (carbon cycle models) or as input
(inversion models). The latitudinal variation of atmospheric CO2 determined
from the NOAA/CMDL Global Air Sampling Network for 198998, with the record from the
South Pole shown in red (Fig. 70), conveys the long-term CO2
increase, the interhemispheric difference, and the seasonal variations of atmospheric CO2.
Note that even though CO2 in the Northern Hemisphere is on average 34 ppm
higher than in the Southern Hemisphere (because 95% of fossil fuel CO2
emissions occur in the Northern Hemisphere), the mean CO2 gradient is reversed
during the northern summer because of photosynthetic uptake by plants. In contrast, the
mean north-south gradient is strongest in spring when respired CO2 has
accumulated in the atmosphere. Carbon cycle models attempt to reproduce this measured
distribution by combining source/sink functions with atmospheric transport models, while
inversion techniques combine the data with a transport model to deduce sources and sinks.
Methane contributes about 20% of the direct radiative forcing
attributed to those long-lived greenhouse gases which are directly affected by human
activities. Changes in the burden of methane also feed back into atmospheric chemistry,
and indirectly affect climate by influencing other greenhouse gases such as tropospheric O3,
stratospheric H2O, and the concentration of OH. These indirect effects are
estimated to add ~40% to the direct climate effect of methane (Lelieveld et al. 1993). It
is suggested that reducing methane emissions to the atmosphere would decrease its
potential effect on climate. However, before reasonable policies can be developed to
reduce CH4 emissions, its budget of sources and sinks must be better
High-precision measurements of atmospheric methane indicate that
approximately 70% of CH4 emissions are in the Northern Hemisphere. As a result,
methane concentrations are also higher in that hemisphere, with the largest concentrations
observed at higher latitudes (Fig. 71). There is a strong
seasonal cycle to methane concentrations throughout the globe, with a particularly regular
seasonal cycle observed in the high latitudes of the Southern Hemisphere (see inset Fig. 71). In the Northern Hemisphere middle and high latitudes
(30°90°N), the seasonal cycle features a peak in CH4 concentrations
from mid-Januarymid-February (Fig. 72a, blue curve) and a
minimum concentration in April.
There is also an upward trend in CH4 concentrations through
the globe, with a de-seasonalized trend in the Northern Hemisphere extratropics of 8.0 ppb
yr1 evident between 198498 (black curve, Fig. 72a) .
However, the instantaneous growth rate of methane in this region, determined as the
derivative of the trend line (Fig. 72b), has decreased by 0.9 ppb
yr-1. Superimposed upon this long-term decrease in growth rate are significant
interannual variations. One outstanding feature is the large increase in growth rate
during 1998, possibly related to increased CH4 emissions from wetlands due to
anomalously warm temperatures in that year. This suggests that changes in climate could
have a large impact on CH4 emissions, and, therefore, on the contribution of
methane to earths radiative budget.
3) Carbon Monoxide
Unlike CO2 and CH4, carbon monoxide is not a
strong absorber in the terrestrial IR spectral region, yet it still has an impact on
climate through its chemistry. The chemistry of CO affects OH (which influences the
lifetimes of CH4 and HFCs) and tropospheric O3 (itself a greenhouse gas), so
emissions of CO can be considered equivalent to emissions of CH4 (Prather
1996). Current emissions of CO may contribute more to radiative forcing over decadal time
scales than do emissions of anthropogenic N2O (Daniel and Solomon 1998).
The lifetime of CO is on the order of weeks to months, depending on
location and season. As a result, CO has large spatial gradients (Fig.
73). At the South Pole, CO varies from about 30 ppb during summer to 60 ppb in winter.
However, CO values are much larger in the Northern Hemisphere where emissions are largest.
For example, at Barrow, Alaska, CO varies from 200 ppb during winter to 90 ppb during
summer. During most of the 1990s, globally-averaged CO amounts decreased by approximately
2% yr-1, possibly due to decreased emissions as a result of catalytic
converters on automobiles (Bakwin et al. 1994). As with CH4 and CO2,
1998 was anomalous in that a significant global increase in CO was observed.
Tropospheric concentrations of total organic equivalent effective
chlorine (EECl) continued to decline through the late 1990s (Fig. 74).
EECl takes both stratospheric release and the increased weighting of bromine into
consideration and essentially represents the ability of the chlorine and bromine in
tropospheric gases to contribute to stratospheric ozone depletion. In the past few years,
CFC11, CFC113, and CCl4 all declined at rates of less than 1% per
year. CFC12, which had been increasing in recent years due to its longer lifetime
and to continued emission from automobile air conditioners, had a 1999 growth rate near
zero in the Northern Hemisphere but continued to increase in the Southern Hemisphere. The
growth of CFC12 concentrations in the Southern Hemisphere results mainly from
The primary gas driving the decrease in total organic chlorine is the
solvent methyl chloroform (CH3CCl3) (Fig. 75).
The Montreal Protocol banned production of CH3CCl3 in developed
countries in 1996, along with that of the CFCs and carbon tetrachloride (CCl4),
another industrial solvent and a precursor to CFC production. Methyl chloroform has a much
shorter lifetime than the CFCs (56 years versus 50120 years), and is
therefore being removed more rapidly from the atmosphere at a rate of more than 15% yr-1.
This rate will decrease in the future as CH3CCl3 is depleted from the atmosphere (Montzka
et al. 1999).
Increases in halon concentrations, particularly for H1211 (CBrClF2)
(Butler et al. 1998; Fraser et al. 1998), are also of concern and underscore the need for
adherence to the Montreal Protocol guidelines if a decline in total chlorine is to
continue. The bromine in these gases has the potential to outweigh the effects of
decreasing chlorine in the atmosphere. The atmospheric burden of HCFCs increased
during 1999 at rates similar to those reported for the early 1990s (Montzka et al. 1996)
In 1999 new information was reported demonstrating that the CFCs,
major chlorocarbons, and halons presently in the atmosphere were absent entirely in the
early 20th century, as shown for air collected in consolidated snow (firn) in the
Greenland Ice Cap (Fig. 76). The gas concentration profiles show
that at this site beyond a depth of 69 m, which relates to CO2 age related
concentrations present in 1929, there was no evidence of any of the CFCs,
chlorocarbons, and halons listed in Figure 76. Measurements of
these gases in air trapped at high Northern and Southern Hemispheric latitudes showed
further that models of anthropogenic emissions could explain the atmospheric histories of
these gases (Butler et al. 1999). This work confirmed that the often-invoked volcanic and
biospheric emissions are inconsequential or nonexistent in their effects upon atmospheric
budgets of these gases, and that the presence of these gases in the atmosphere is due
almost entirely to human sources.