Cause, conseguenze e strategie di mitigazione Proponiamo il primo di una serie di articoli in cui affronteremo l’attuale problema dei mutamenti climatici. Presentiamo il documento redatto, votato e pubblicato dall’Ipcc - Comitato intergovernativo sui cambiamenti climatici - che illustra la sintesi delle ricerche svolte su questo tema rilevante.

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Climate Change 2007: The Physical Science Basis.

The Pacific Decadal Oscillation (PDO) has been described by some as a long-lived El Nino-like pattern of Pacific climate variability, and by others as a blend of two sometimes independent modes having distinct spatial and temporal characteristics of North Pacific sea surface temperature (SST) variability. A growing body of evidence highlights a strong tendency for PDO impacts in the Southern Hemisphere, with important surface climate anomalies over the mid-latitude South Pacific Ocean, Australia and South America. Several independent studies find evidence for just two full PDO cycles in the past century: “cool” PDO regimes prevailed from 1890–1924 and again from 1947–1976, while “warm” PDO regimes dominated from 1925–1946 and from 1977 through (at least) the mid-1990's. Interdecadal changes in Pacific climate have widespread impacts on natural systems, including water resources in the Americas and many marine fisheries in the North Pacific. Tree-ring and Pacific coral based climate reconstructions suggest that PDO variations—at a range of varying time scales—can be traced back to at least 1600, although there are important differences between different proxy reconstructions. While 20th Century PDO fluctuations were most energetic in two general periodicities—one from 15-to-25 years, and the other from 50-to-70 years—the mechanisms causing PDO variability remain unclear. To date, there is little in the way of observational evidence to support a mid-latitude coupled air-sea interaction for PDO, though there are several well-understood mechanisms that promote multi-year persistence in North Pacific upper ocean temperature anomalies.

https://link.springer.com/content/pdf/10.1023%2FA%3A1015820616384.pdf

The Pacific Decadal Oscillation

North Atlantic sea surface temperatures for 1856-1999 contain a 65-80 year cycle with a 0.4 C range, referred to as the Atlantic Multidecadal Oscillation (AMO) by Kerr (2000). AMO warm phases occurred during 1860- 1880 and 1940-1960, and cool phases during 1905-1925 and 1970-1990. The signal is global in scope, with a posi- tively correlated co-oscillation in parts of the North Pa- cic, but it is most intense in the North Atlantic and cov- ers the entire basin there. During AMO warmings most of the United States sees less than normal rainfall, including Midwest droughts in the 1930s and 1950s. Between AMO warm and cool phases, Mississippi River outflow varies by 10% while the inflow to Lake Okeechobee, Florida varies by 40%. The geographical pattern of variability is influenced mainly by changes in summer rainfall. The winter patterns of interannual rainfall variability associated with El Ni~no- Southern Oscillation are also signicantly changed between AMO phases.

/pdf/the-atlantic-multidecadal-oscillation-and-its-relation-to-2zsepunpa6.pdf

The Atlantic Multidecadal Oscillation and its relation to rainfall and river flows in the continental U.S.

Infectious diseases can cause rapid population declines or species extinctions. Many pathogens of terrestrial and marine taxa are sensitive to temperature, rainfall, and humidity, creating synergisms that could affect biodiversity. Climate warming can increase pathogen development and survival rates, disease transmission, and host susceptibility. Although most host-parasite systems are predicted to experience more frequent or severe disease impacts with warming, a subset of pathogens might decline

/pdf/climate-warming-and-disease-risks-for-terrestrial-and-marine-gttfcx5ulf.pdf

Climate Warming and Disease Risks for Terrestrial and Marine Biota

http://www.highstand.org/erohling/Rohling-papers/2004-Mayewski%20et%20al%20Holocene%20climate%20variability%20QR62.pdf

Holocene climate variability

Past analyses of tropical Atlantic sea surface temperature variability have suggested a dipole behavior between the northern and southern tropics, across the Intertropical Convergence Zone (ITCZ). By analyzing an improved 43-year (1950-1992) record of SST (Smith et al., 1996) and other data derived from the Comprehensive Ocean-Atmosphere Data Set (COADS), it is shown that the regions north and south of the ITCZ are statistically independent of each other at the seasonal to interannual timescales dominating the data, confirming the conclusions of Houghton and Tourre (1992). Some dipole behavior does develop weakly during the boreal spring season, when there is a tendency for SST anomaly west of Angola to be opposite of that in the tropical North Atlantic. It is further shown that tropical Atlantic SST variability is corre- lated with Pacific E1 Nifio-Southern Oscillation (ENSO) variability in several regions. The ma- jor region affected is the North Atlantic area of NE trades west of 40oW along 10oN - 20oN and extending into the Caribbean. There, about 50-80% of the anomalous SST variability is associ- ated with the Pacific ENSO, with Atlantic warmings occurring 4-5 months after the mature phases of Pacific warm events. An analysis of local surface flux fields derived from COADS data shows that the ENSO-related Atlantic warmings occur as a result of reductions in the surface NE trade wind speeds, which in turn reduce latent and sensible heat losses over the region in ques- tion, as well as cooling due to entrainment. This ENSO connection is best developed during the boreal spring following the most frequent season of maximum ENSO anomalies in the Pacific. A region of secondary covariability with ENSO occurs along the northern edge of the mean ITCZ position and appears to be associated with northward migrations of the ITCZ when the North At- lantic warmings occur. Although easterly winds are intensified in the western equatorial Atlantic in response to Pacific warm events, they do not produce strong local changes in SST. Contrary to expectations from studies based on equatorial dynamics, these teleconnected wind anomalies do not give rise to significant correlations of SST in the Gulf of Guinea with the Pacific ENSO. As the teleconnection sequence matures, strong SE trades at low southern latitudes follow the de- velopment of the North Atlantic SST anomaly and precede by several months the appearance of weak negative SST anomalies off Angola and stronger positive anomalies extending eastward from southern Brazil along 15o-30oS.

http://www.o3d.org/abracco/Atlantic/atlantic_enso/enfield_mayer_1997.pdf

Tropical Atlantic sea surface temperature variability and its relation to El Niño‐Southern Oscillation

Debris flows have deposited inorganic laminae in an alpine lake that is 75 kilometers east of the Pacific Ocean, in Ecuador. These storm-induced events were dated by radiocarbon, and the age of laminae that are less than 200 years old matches the historic record of El Nino events. From about 15,000 to about 7000 calendar years before the present, the periodicity of clastic deposition is greater than or equal to 15 years; thereafter, there is a progressive increase in frequency to periodicities of 2 to 8.5 years. This is the modern El Nino periodicity, which was established about 5000 calendar years before the present. This may reflect the onset of a steeper zonal sea surface temperature gradient, which was driven by enhanced trade winds.

https://science.sciencemag.org/content/sci/283/5401/516.full.pdf

An ;15,000-Year Record of El Nino—Driven Alluviation in Southwestern Ecuador

The behavior and relationship of anomalies of monthly mean sea level, coastal sea surface temperature and alongshore wind stress for the eastern Pacific Ocean during the period 1950–74 have been studied Sea level and temperature records from Yakutat, Alaska (59°N) to Valparaiso, Chile (33°S) and computed alongshore wind stress at near coastal grid points from Yakutat to Matzatlan, Mexico (23°N) have been utilized The positive and negative sea level anomalies, corresponding to El Nino-anti El Nino cycles, are well correlated throughout the tropics of both hemispheres and are detectable at the California stations From Crescent City to Antofagasta, Chile the sea level anomalies were correlated with the Southern Oscillation Index above the 99% significance level The maximum station separations for which sea level anomalies were correlated among themselves above the 99% significance level varied from 6000 km (Yakutat to San Diego) to more than 12 000 km (Prince Rupert to Matarani) A well-defined 

/pdf/on-the-structure-and-dynamics-of-monthly-mean-sea-level-48eu5dhdt5.pdf

On the Structure and Dynamics of Monthly Mean Sea Level Anomalies along the Pacific Coast of North and South America

Several kinds of analysis are applied to sea surface temperature anomalies (SSTA) (1856–1991) to determine the degree to which SSTA of opposite sign in the tropical North and South Atlantic occur. Antisymmetric (“dipole”) configurations of SSTA on basin scales are not ubiquitous in the tropical Atlantic. Unless the data are stratified by both season and frequency, inherent dipole behavior cannot be demonstrated. Upon removing the global El Nino-Southern Oscillation signal in SSTA (which is symmetric between the North and South Atlantic) from the data, the regions north or south of the Intertropical Convergence Zone have qualitatively different temporal variabilities and are poorly correlated. Dipole configurations do occur infrequently (12–15% of the time), but no more so than expected by chance for stochastically independent variables. Nondipole configurations that imply significant meridional SSTA gradients occur much more frequently, nearly half of the time. Cross-spectral analysis of seasonally averaged SSTA indices for the North and South Atlantic show marginally significant coherence with antisymmetric phase in two period bands: 8–12 years for the boreal winter-spring and 2.3 years for the boreal summer-fall. Antisymmetric coherence is optimal for a small subregion west of Angola in the South Atlantic, with respect to SSTA of basin scale in the tropical North Atlantic. Dipole variability, even where optimal, explains only a small fraction of the total variance in tropical Atlantic SSTA (<7%).

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1998JC900109

David B. Enfield

Papers

The Atlantic Multidecadal Oscillation and its relation to rainfall and river flows in the continental U.S.

Tropical Atlantic sea surface temperature variability and its relation to El Niño‐Southern Oscillation

An ;15,000-Year Record of El Nino—Driven Alluviation in Southwestern Ecuador

On the Structure and Dynamics of Monthly Mean Sea Level Anomalies along the Pacific Coast of North and South America

How ubiquitous is the dipole relationship in tropical Atlantic sea surface temperatures