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07 Is UV-B at the earth's surface increasing?




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This article is from the Ozone Depletion: UV Radiation and its Effects FAQ, by Robert Parson rparson@spot.colorado.edu with numerous contributions by others.

07 Is UV-B at the earth's surface increasing?

Yes, in some places; no, in some others; unknown, in most.

There is very little data on long-term UV trends, primarily because
with very few exceptions UV monitoring operations of the requisite
sensitivity did not exist until very recently. (See the US
Department of Agriculture's UV Monitoring Program web page,
http://uvb.nrel.colostate.edu/UVB/uvb_climate_network.html.)
Measurements over a period of a few years cannot establish long-term
trends, although they can be used in conjunction with ozone measurements
to quantify the relationship between surface UV-B intensities and
ozone amounts.

Very large increases, by as much as a factor of 2-3, have been seen
within the Antarctic ozone hole. [Frederick and Alberts] [Stamnes et
al.] UV-B intensity at Palmer Station (65 degrees S. Lat.) in late
October 1993 exceeded *summertime* UV-B intensity at San Diego,
California. [WMO 1994] At Ushaia at the tip of South America, the
noontime UV-B irradiance in the austral summer is 45% above what would
be predicted were there no ozone depletion. [Frederick et al. 1993]
[Bojkov et al. 1995] The effect is to expose Ushaia to UV intensities
that are typical of Buenos Aires.

Small increases, of order 1% per year, have been measured in the
Swiss Alps. [Blumthaler and Ambach] These _net_ increases are small
compared to natural day-to-day fluctuations, but they are actually
a little larger than would be expected from the amount of ozone
depletion over the same period.

In urban areas of the US, measurements of erythemal UV-B showed no
significant increase (and in most cases a slight decrease between 1974 and
1985. [Scotto et al.]. This may be due due to increasing urban
pollution, including low-level ozone and aerosols. [Grant]
Tropospheric ozone is actually somewhat more effective at absorbing UV
than stratospheric ozone, because UV light is scattered much more in
the troposphere, and hence takes a longer path. [Bruehl and Crutzen]
Increasing amounts of tropospheric aerosols, from urban and industrial
pollution, may also offset UV-B increases at the ground. [Liu et al.]
[Madronich 1992, 1993] [Grant] There have been questions about the
suitability of the instruments used by Scotto et al.; they were not
designed for measuring long-term trends, and they put too much weight
on regions of the UV spectrum which are not appreciably absorbed by
ozone in any case. [WMO 1989] A thorough reassessment
[Weatherhead et al. 1997] found a number of problems:

"The RB meter network was originally established to determine the
relative amounts of UV at different locations around the earth,
with most sites in the United States. The data have been useful for
their intended purpose, that is, to help explain differences in skin
cancer at different locations. There was no original plan to use
the network to determine trends, and therefore the network was not
maintained using the high level of standards necessary for accurate
trend determination. The network management, calibration techniques,
and in some cases instrument location, underwent changes over the
20 years of operation. Unfortunately, most of the records documenting
the maintenance and calibration of the network were misplaced during
transfer of the network among different managers."

Nevertheless it seems clear that so far
ozone depletion over US cities is small enough to be largely offset by
competing factors. Tropospheric ozone and aerosols have increased in
rural areas of the US and Europe as well, so these areas may also be
screened from the effects of ozone depletion.

Several studies [Kerr and McElroy] [Seckmayer et al.] [Zerefos et
al.] have presented evidence of short-term UV-B increases at northern
middle latitudes (Canada, Germany, and Greece), associated with the
record low ozone levels seen in these areas in the years 1992-93. As
discussed in Part I, these low ozone levels are probably due to
stratospheric sulfate aerosols from the 1991 eruption of Mt.Pinatubo;
such aerosols change the radiation balance in the stratosphere,
influencing ozone production and transport, and accelerate the
conversion of inactive chlorine reservoir compounds into
ozone-destroying ClOx radicals. The first mechanism is purely natural,
while the second is an example of a natural process enhancing an
anthropogenic mechanism since most of the chlorine comes ultimately
from manmade halocarbons. (High UV levels associated with low ozone
levels were also reported in Texas [Mims 1994, Mims et al. 1995],
however in this case the low ozone is attributed to unusual
climatology rather than chemical ozone destruction.) One cannot
deduce long-term trends from such short-term measurements, but one can
use them to help quantify the relationship between stratospheric ozone
and surface UV-B intensities under real world conditions. Measurements
in Toronto, Canada [Kerr and McElroy] over the period 1989-93 found
that UV intensity at 300 nm increased by 35% per year in winter and 7%
per year in summer. At this wavelength 99% of the total UV is
absorbed, so these represent large increases in a small number, and do
not represent a health hazard; nevertheless these wavelengths play a
disproportionately large role in skin carcinoma and plant damage.
_Total_ UV-B irradiance, weighted in such a way as to correlate with
incidence of sunburn ("erythemally active radiation"), increased by 5%
per year in winter and 2% per year in summer. These are not really
"trends", as they are dominated by the unusually large, but temporary,
ozone losses in these regions in the years 1992-1993 (see part I), and
they should not be extrapolated into the future. Indeed, [Michaels et
al.] have claimed that the winter "trend" arises entirely from a brief
period at the end of March 1993 (they do not discuss the summer
trend.) Kerr and McElroy respond that these days are also reponsible
for the strong decrease in average ozone over the same period, so that
their results do demonstrate the expected link between total ozone and
total UV-B radiation. UV-B increases of similar magnitude were seen
in Greece for the period 1990-1993 [Zerefos et al.] and in Germany
for the period 1992-93. [Seckmeyer et al.]

Indirect evidence for increases has been obtained in the Southern
Hemisphere, where stratospheric ozone depletion is larger and
tropospheric ozone (and aerosol pollution) is lower. Biologically
weighted UV-B irradiances at a station in New Zealand were 1.4-1.8
times higher than irradiances at a comparable latitude and season in
Germany, of which a factor of 1.3-1.6 can be attributed to differences
in the ozone column over the two locations [Seckmeyer and McKenzie].

Record low ozone columns measured at Mauna Loa during the winter
of 1994-95 were accompanied by corresponding increases in the ratio
of UV-B to UV-A [Hofmann et al. 1996.]

The satellite-borne Total Ozone Mapping Spectrometer (TOMS) actually
measures the UV radiation that is scattered back into space from the
earth's atmosphere. [Herman et al. 1996] have combined ozone and
reflectivity data from TOMS with radiative transfer calculations to
arrive at an estimate of the ultraviolet flux at the surface. The
estimates are validated by comparison with ground-based UV measurements.
The advantage of this technique is that it gives truly global
coverage; the disadvantage is that it is indirect. Herman et al.
estimate that during the period 1979-92 UV irradiance, weighted for
DNA damage, increased by ~5% per decade at 45 degrees N latitude,
~7% per decade at 55 N, and ~10% per decade at 55 S. The increases
occurred primarily in spring and early summer.

 

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