Climate change futures: health, ecological and economic dimensions

While the role of allergen exposure alone in causing
allergic diseases is unknown, allergens from pollen
grains and fungal spores are unequivocally associated
with exacerbation of existing disease. Changes in
atmospheric chemistry and climate that tend to
increase the presence of pollen and fungi in the air
therefore contribute to a heightened risk of allergic
symptoms and asthma.
THE ROLE OF CLIMATE
POLLEN
Several studies have explored the potential impacts of
CO 2
and global warming on plants. In general,
increasing CO 2
and temperatures stimulate plants to
increase photosynthesis, biomass, water-use efficiency
and reproductive effort (Bazzaz 1990; LaDeau and
Clark 2001). These are considered positive responses
for agriculture, but for allergic individuals, they could
mean increased exposure to pollen allergens.
Figure 2.13 Pollen Grains
Electron microscope image of ragweed pollen grains.
Image: Johns Hopkins University
Second, recent studies have shown increased plant
reproductive effort and pollen production under conditions
of elevated CO 2
. For example, loblolly pines
respond to experimentally elevated CO 2
(200 ppm
above ambient levels) by tripling their production of
seeds and cones (LaDeau and Clark 2001). Long-term
records at pollen monitoring stations in Europe show
increasing annual pollen totals for several types of
trees including hazel, birch and grasses (Spieksma
et al. 1995; Frei 1998).
For ragweed (Ambrosia artemisiifolia), a weed of
open disturbed ground that produces potent pollen
allergens, controlled-environment experiments show
that plants grown at two times ambient CO 2
have
greater biomass, and produce 40% to 61% more
pollen than do controls (Ziska and Caulfield 2000;
Wayne et al. 2002) (see figure 2.13). This finding
was corroborated in field experiments using a natural
urban-rural CO 2
gradient (Ziska et al. 2003).
Figure 2.14 Ragweed Pollen Production Under Elevated CO 2
Percent increase above controls
ragweed growth
70
60
50
40
30
20
10
0
350 ppm CO 2
700 ppm CO
Carbon dioxide concentration
Height Seed mass Pollen Biomass
2
49 | INFECTIOUS AND RESPIRATORY DISEASES
Beyond these broad changes, climate warming has
resulted in specific phenological changes in plants.
First, the timing of spring budding has advanced in
recent decades (Fitter and Fitter 2002; van Vliet et al.
2003). Hence the allergenic pollen season for many
spring flowering plants also begins earlier (Jäger et al.
1996; Frei 1998; van Vliet et al. 2002). The rate of
these advances is 0.84–0.9 days/year (Frenguelli et
al. 2002; Clot 2003). While this is generally considered
to be solely the effect of temperature, some studies
suggest that CO 2
can independently affect the timing
of phenological events as well (Murray and
Ceulemans 1998).
Plants grown at high CO 2 levels grow moderately more (9%) but
produce significantly more (61%) pollen than those grown at lower
levels. Source: Wayne et al. 2002
Increased temperature and CO 2
can also have interactive
effects on pollen production due to longer growing
seasons. In experiments simulating early spring, ragweed
plants grew larger, had more flowers and produced
more pollen than did plants started later. Those
started later with high CO 2
produced 55% more
pollen than did those grown at ambient CO 2
(Rogers
et al. in review).
CASE STUDIES