ScienceDaily, February 24, 2012
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| The projections are based on various scenarios for future HFC emissions. In the best case, in which HFC emissions will be as low as future CFC emissions, the current mix of HFCs (with an average atmosphere lifetime of 15 years and an average global warming potential of 1600) will be replaced by more "ecological" HFCs with an average lifetime of less than 2 months and a global warming potential of less than 20. (Credit: HFCs: A Critical Link in Protecting Climate and the Ozone Layer, UNEP, 2011) |
The Montreal
Protocol led to a global phase-out of most substances that deplete the ozone
layer, such as chlorofluorocarbons (CFCs). A happy side-effect of the gradual
ban of these products is that Earth's climate has also benefited because CFCs
are also potent greenhouse gases. However, now a "rebound effect"
threatens to accelerate the rate of global warming.
Hydrofluorocarbons
(HFCs), which have been used in recent years in increasing quantities as
substitutes for CFCs, are also climatically very active and many are also
extremely long-lived. In the journal Science an international team of
researchers recommends that the most potent of these gases also be regulated.
This could save the positive "side effect" of the Montreal Protocol
for the global climate.
It is regarded as
the most successful international environmental agreement and has, to date,
been ratified by 196 countries -- the Montreal Protocol on Substances That
Deplete the Ozone Layer. As a result, CFCs and ozone "killers" will
gradually disappear from the atmosphere over the coming decades. And because
many of these substances are also very active greenhouse gases, Earth's climate
will profit from the sinking concentrations too.
So far, so good. In
many processes where previously CFCs were used, these are now being increasingly
substituted by fluorinated compounds such as HFCs (which, simply put, are
similar substances to CFCs but do not contain chlorine and do not deplete
stratospheric ozone). They are used as cooling agents in air conditioning
plants and refrigerators, as propellants in aerosol cans, as solvents and as
foaming agents in the manufacture of foam products. However, there is a
downside to the use of HFCs -- they are also very potent greenhouse gases.
HFC-134a, also known as R-134a, for example, which is used in automobile air
conditioning units, is 1430 more active than the "classic" greenhouse
gas carbon dioxide (CO2).
International
environmental agreements can also have unwanted side effects
The reduction of
greenhouse gas emissions is covered by the Kyoto Protocol. This agreement is,
however, not binding for the world's largest emitter of greenhouse gases, the
USA (which has never ratified the protocol), nor for threshold and developing
countries. In addition the Kyoto Protocol is currently limited to the period
from 2000 to 2012. No agreement has yet been reached on extending it. What this
means is that the significant increase in global emissions of HFCs seen over
the past few years will soon negate the positive effects on climate brought by
the Montreal protocol's CFC phase-out.
This link is shown
by an analysis published in the latest issue of "Science." An
international team of researchers, headed by Holland's Guus Velders and
including the chemistry Nobel laureate Mario Molina and Empa researcher Stefan
Reimann, investigated the unintentional (positive) climate effects resulting
from the Montreal Protocol. Since the year 2000 the radiative forcing (a
measure of the effect on the climate of chemical substances) of all
ozone-depleting substances including CFCs has remained at a more or less
constant value of 0.32 W/m2, compared to a value of 1.5 W/m2
for CO2. Had the Montreal Protocol recommendations not been
implemented, today's value would be approximately double this figure, i.e. 0.65
W/m2. Putting things another way, the CFC ban has prevented the
equivalent of 10 billion tonnes of CO2 being emitted into the
atmosphere in 2010, five times the annual reduction target set by the Kyoto
Protocol.
Velders, Reimann
and their co-authors fear that this positive effect will soon be negated by HFC
emissions, which are currently increasing at 10 to 15% annually. In their
article they state that "the HFC contribution to climate change can be
viewed as an unintended negative side effect" of the Montreal Protocol. At
the moment the effect is still small -- about 0.012 W/m2 for all CFC
substitutes combined. But it is beyond question that radiative forcing due to
HFCs will rise significantly in future as a result of increasing demand and
production for these substances, above all in threshold and developing
countries. The atmospheric scientists estimate that this value will rise to
between 0.25 und 0.4 W/m2 by the year 2050. The greatest problem is
presented by saturated HFCs, which are extremely stable and survive in the
atmosphere for up to 50 years, exhibiting a long-term global warming potential
of up to 4000 times higher than CO2. For Empa researcher Reimann the
situation is clear: "Long-lived HFCs should no longer be used in these
quantities."
A
"simple" solution: expanding the scope of the Montreal Protocol
Among
other things, the scientists recommend modifying the Montreal protocol so that
it also covers the use of long-lived HFCs. Such proposals have already been
tabled in previous years by various countries including the USA. "Since it
is it is as a result of the Montreal protocol that these substances are being
manufactured in increasing amounts, they could be included in the agreement
too, so their use can be regulated as well," maintains Reimann. A stepwise
phase-out of HFCs is technically feasible since, according to Reimann, chemical
and technological alternatives are already available. In the USA for example
refrigerators are cooled using HFC-134a; in Switzerland the use of this
substance in refrigerators is banned and climate neutral hydrocarbons are used
instead.
Story Source:
The above story is reprinted from materials provided by Empa, via AlphaGalileo.Note: Materials may be edited for content and length. For further information, please contact the source cited above.
Journal Reference:
- G. J. M. Velders, A. R. Ravishankara, M. K. Miller, M. J. Molina, J. Alcamo, J. S. Daniel, D. W. Fahey, S. A. Montzka, S. Reimann. Preserving Montreal Protocol Climate Benefits by Limiting HFCs. Science, 2012; 335 (6071): 922 DOI: 10.1126/science.1216414
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