270
MJ. Esteban-Parra, D. Pow-Vazquez, F.S. Rodrigo, Y. Castro-Dfez
in this case. Other researchers also argue that the temperature rise cannot be
attributed solely to urbanization processes, due to the presence of this warming in
sites with different development of the urbanization and land use (Palmieri et aI.,
1991).In fact, some localities, such as Segovia, with only 53.000 inhabitants, have
increases higher than in Madrid, with more than 3.000.000 inhabitants.
Currently, one of the leading aims in the research on climatic change is not only
to detect the anthropogenic effect, but also to quantify this effect and also to
determine how much of the change is due to the natural climatic variability. One
behavior. possible agent of the natural variability is the volcanism. In general,
volcanic eruptions bring about short periods of cooling, by expelling sulphur gases
and aerosols into the atmophere. At the end ofthe last century, volcanic activity was
quite intense, exemplified by the stupendous Krakatoa (1883)and Tarawera (1886)
eruptions, which influenced meteorological records of continental Europe (Bradley,
1988). Another major eruption was Pinatubo, in Philippines in June of 1991, which
may be associated with the relative cold years in the last warm period (1991 began
to be colder in autumn, when the Pinatubo effect arrived Spain (Olmo and Alados,
1995).
We cannot attribute all the warming detected to the greenhouse effect, but the
comparative study of some of the fluctuations can help to define the magnitude of
the effect, but following Wallace et aI. (1995) arguments, for Spain, radiativeforcing
of the temperature climate could be the main responsible of the behaviour found.
The difference between the last century and the beginning of this one can be
considered natural fluctuation, perhaps related to the volcanic activity and the end
of the Little Ice Age at the end of last century, provided the greenhouse effect was
minor during this epoch (Bradley and Jones, 1995). From this point of view lower
temperatures in the 1970s may reflect the high magnitude of the natural variability,
and nullify (at least for local areas such as the Iberian Peninsula and Balearic
Islands) the greenhouse effect during certain periods. The strong increase in the
1980 could be provoked by anthropogenic causes, natural variability or both. The
first possibility could be related to an abrupt response of the climatic system to the
human forcing, although it is the cold decade ofthe 1970 s which breaks the trends.
Despite of the powerful eruption of EI Chich6n in 1982 and its cooling effects, the
1980s generally had the highest temperatures, the maximum temperature being for
1989 (prior to that period only isolated years present very high values). Thus, if the
increase was due only to natural variability, it would be the first instance of this
strong value and persistence. Given the relative good agreement with the global and
hemispheric series, and that these series are relatively well represented by the
climate models (IPCC, 1995), this increase may in fact be partially due to the
increased greenhouse effect, as some researchers believe (IPCC, 1995). This
hypothesis includes that the increasing trend will continue in the future.
MJ. Esteban-Parra, D. Pow-Vazquez, F.S. Rodrigo, Y. Castro-Dfez
in this case. Other researchers also argue that the temperature rise cannot be
attributed solely to urbanization processes, due to the presence of this warming in
sites with different development of the urbanization and land use (Palmieri et aI.,
1991).In fact, some localities, such as Segovia, with only 53.000 inhabitants, have
increases higher than in Madrid, with more than 3.000.000 inhabitants.
Currently, one of the leading aims in the research on climatic change is not only
to detect the anthropogenic effect, but also to quantify this effect and also to
determine how much of the change is due to the natural climatic variability. One
behavior. possible agent of the natural variability is the volcanism. In general,
volcanic eruptions bring about short periods of cooling, by expelling sulphur gases
and aerosols into the atmophere. At the end ofthe last century, volcanic activity was
quite intense, exemplified by the stupendous Krakatoa (1883)and Tarawera (1886)
eruptions, which influenced meteorological records of continental Europe (Bradley,
1988). Another major eruption was Pinatubo, in Philippines in June of 1991, which
may be associated with the relative cold years in the last warm period (1991 began
to be colder in autumn, when the Pinatubo effect arrived Spain (Olmo and Alados,
1995).
We cannot attribute all the warming detected to the greenhouse effect, but the
comparative study of some of the fluctuations can help to define the magnitude of
the effect, but following Wallace et aI. (1995) arguments, for Spain, radiativeforcing
of the temperature climate could be the main responsible of the behaviour found.
The difference between the last century and the beginning of this one can be
considered natural fluctuation, perhaps related to the volcanic activity and the end
of the Little Ice Age at the end of last century, provided the greenhouse effect was
minor during this epoch (Bradley and Jones, 1995). From this point of view lower
temperatures in the 1970s may reflect the high magnitude of the natural variability,
and nullify (at least for local areas such as the Iberian Peninsula and Balearic
Islands) the greenhouse effect during certain periods. The strong increase in the
1980 could be provoked by anthropogenic causes, natural variability or both. The
first possibility could be related to an abrupt response of the climatic system to the
human forcing, although it is the cold decade ofthe 1970 s which breaks the trends.
Despite of the powerful eruption of EI Chich6n in 1982 and its cooling effects, the
1980s generally had the highest temperatures, the maximum temperature being for
1989 (prior to that period only isolated years present very high values). Thus, if the
increase was due only to natural variability, it would be the first instance of this
strong value and persistence. Given the relative good agreement with the global and
hemispheric series, and that these series are relatively well represented by the
climate models (IPCC, 1995), this increase may in fact be partially due to the
increased greenhouse effect, as some researchers believe (IPCC, 1995). This
hypothesis includes that the increasing trend will continue in the future.