Registro de temperatura del último milenio

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Información descriptiva del período cálido medieval y de la pequeña Edad de Hielo en varios reportes IPCC, ver Descripción del período cálido medieval y la pequeña edad de hielo en los informes del IPCC
Reconstrucciones de las Tº en el Hemisferio Norte del 2º milenio, a tono con varios artículos antiguos (líneas azuladas), más nuevos (líneas rojizas), y registro instrumental (línea negra)

El registro de temperaturas en el II milenio describe la reconstrucción de Tª desde el año 1000 en el Hemisferio Norte, y posteriormente extendido hasta el año 1, y luego también cubriendo el hemisferio sur. Es necesaria una reconstrucción debido a contar con un registro confiable de la Tª superficial, sólo desde 1850. Estudiar el clima del pasado es de interés para los científicos, pues mejora la comprensión de la variabilidad climática actual y, relacionado con lo anterior, proporciona una mejor base para las proyecciones futuras del clima. En particular, si la naturaleza y la magnitud de la variabilidad natural del clima puede ser establecida, los científicos serán capaces de detectar en mejor manera las contribuciones antropogénicas calentamiento global. Cabe aclarar que, no obstante, a pesar de que la reconstrucción de las temperaturas obtenidas desde los datos que brindan los proxis nos ayudan a entender las características de la variación del clima natural, esto se basa en una amplia variedad de metodologías en las que las reconstrucciones de proxi son sólo una pequeña parte.[1] [2]

De acuerdo con todas las reconstrucciones de temperatura importantes publicados en revistas revisadas por pares (ver gráfico), el incremento en la temperatura en el siglo XX y la temperatura a finales de este siglo es la más alta en los registros. La atención ha tendido a centrarse en los primeros trabajos de Michael E. Mann, Bradley and Hughes (1998), cuyo "palo de hockey" gráfico fue presentado en el 2001 en el reporte de la United Nations Intergovernmental Panel on Climate Change. La metodología y los conjuntos de datos usados para la creación del the Mann et al. (1998) versión del gráfico del palo de hockey están en controversia con Stephen McIntyre y Ross McKitrick, aunque el gráfico está en general aceptado por la comunidad científica.

Registro instrumental de Tº de los últimos 150 años

Técnicas generales y precisión[editar]

Sin lugar a dudas el mejor período observado es que el que va desde 1850 hasta la actualidad, con la cobertura mejorada con el tiempo. A lo largo de este período los instrumentos para medir la temperatura, mayormente basados en la lectura directa de los termómetros, tiene aproximadamente cobertura global. Esto muestra un incremento general de las temperaturas a nivel global.

Antes de este tiempo deben ser usados varios proxis. Estos proxis son menos exactos que la lectura directa de los termómetros, tienen una menor resolución temporal y menor cobertura espacial. Su única ventaja es que permiten una reconstrucción de los registros más prolongada. Desde que los registros de temperaturas directos son más precisos que los proxis (en efecto, son necesarios para calibrarlos) son usados cuando están disponibles, por ejemplo, de 1850 en adelante.

Métodos cuantitativos a partir de datos proxy[editar]

As there are few instrumental records before 1850, temperatures before then must be reconstructed based on proxy methods. One such method, based on principles of dendroclimatology, uses the width and other characteristics of tree rings to infer temperature. The isotopic composition of snow, corals, and stalactites can also be used to infer temperature. Other techniques which have been used include examining records of the time of crop harvests, the treeline in various locations, and other historical records to make inferences about the temperature. These proxy reconstructions are indirect inferences of temperature and thus tend to have greater uncertainty than instrumental data.

In general, the recent history of the proxy records is calibrated against local temperature records to estimate the relationship between temperature and the proxy. The longer history of the proxy is then used to reconstruct temperature from earlier periods. Proxy records must be averaged in some fashion if a global or hemispheric record is desired. Considerable care must be taken in the averaging process; for example, if a certain region has a large number of tree ring records, a simple average of all the data would strongly over-weight that region. Hence data-reduction techniques such as principal components analysis are used to combine some of these regional records before they are globally combined. An important distinction is between so-called 'multi-proxy' reconstructions, which attempt to obtain a global temperature reconstructions by using multiple proxy records distributed over the globe and more regional reconstructions. Usually, the various proxy records are combined arithmetically, in some weighted average. More recently, Osborn and Briffa used a simpler technique, counting the proportion of records that are positive, negative or neutral in any time period.[3] [4] This produces a result in general agreement with the conventional multi-proxy studies.

Several reconstructions suggest there was minimal variability in temperatures prior to the 20th century (see, for example, [2]). More recently, Mann and Jones have extended their reconstructions to cover the 1st and 2nd millennia (GRL, 2003[5] ). The work was reproduced by Wahl and Ammann in 2005.[6]

The Mann, Bradley and Hughes (1998) version of the temperature record is known as the "Hockey Stick" graph, first coined by Jerry Mahlman, director of the Geophysical Fluid Dynamics Laboratory.

The work of Mann et al., Jones et al., Briffa and others[7] [8] forms a major part of the IPCC TAR's conclusion that "the rate and magnitude of global or hemispheric surface 20th century warming is likely to have been the largest of the millennium, with the 1990s and 1998 likely to have been the warmest decade and year".[9] The IPCC AR4 concluced that "Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1,300 years".[10]

Reconstrucción cualitativa con los registros históricos[editar]

It is also possible to use historical data such as times of grape harvests, sea-ice-free periods in harbours and diary entries of frost or heatwaves to produce indications of when it was warm or cold in particular regions. These records are harder to calibrate, are often only available sparsely through time, may be available only from developed regions, and are unlikely to come with good error estimates. These historical observations of the same time period show periods of both warming and cooling.

El astrofísico Sallie Baliunas notes that these temperature variations correlate with solar variation[11] and asserts that the number of observed sunspots give us a rough measure of how bright the sun is. Balunias and others have suggested that periods of decreased solar radiation are partially responsible for historically recorded periods of cooling such as the Maunder Minimum and the Little Ice Age. The same argument would imply that periods of increased solar radiation contributed to the Medieval Warm Period, when Greenland's icy coastal areas thawed enough to permit farming and colonisation.

Limitaciones[editar]

The apparent differences between the quantitative and qualitative approaches are not fully reconciled. The reconstructions mentioned above rely on various assumptions to generate their results. If these assumptions do not hold, the reconstructions would be unreliable. For quantitative reconstructions, the most fundamental assumptions are that proxy records vary with temperature and that non-temperature factors do not confound the results. In the historical records temperature fluctuations may be regional rather than hemispheric in scale.

In a letter to Nature (10 de agosto 2006) Bradley, Hughes and Mann[12] pointed at the original title of their 1998 article: Northern Hemisphere temperatures during the past millennium: inferences, uncertainties, and limitations[13] and pointed out more widespread high-resolution data are needed before more confident conclusions can be reached and that the uncertainties were the point of the article.

Controversia del gráfico de hockey[editar]

There is an ongoing debate about the details of the temperature record and the means of its reconstruction, centered on the Mann, Bradley and Hughes (1998), "hockey stick" graph. Stephen McIntyre and Ross McKitrick[14] claimed various errors in the methodology of Mann et al. (1998) and that the method of Mann, Bradley, and Hughes "when tested on persistent red noise, nearly always produces a hockey stick shaped first principal component". In turn, Michael E. Mann (supported by Tim Osborn, Keith Briffa and Phil Jones of the Climatic Research Unit) have disputed the claims made by McIntyre and McKitrick.[15] [16] The IPCC Fourth Assessment Report says that McIntyre and McKitrick "may have some theoretical foundation, but Wahl and Ammann (2006)[6] also show that the impact on the amplitude of the final reconstruction is very small (~0.05 °C)."[17]

Véase tambinen[editar]

Referencias[editar]

  1. Houghton, 2001 12. Detection of Climate Change and Attribution of Causes
  2. «What If … the "Hockey Stick" Were Wrong?». RealClimate (27 de enero de 2005).
  3. Osborn, T. J.; Briffa, K. R. (2006). «The Spatial Extent of 20th-Century Warmth in the Context of the Past 1200 Years». Science (AAAS) 311 (5762):  pp. 841–844. doi:10.1126/science.1120514. PMID 16469924. Bibcode2006Sci...311..841O. http://www.sciencemag.org/cgi/content/abstract/311/5762/841. 
  4. «A New Take on an Old Millennium». RealClimate (9 de febrero de 2006).
  5. Mann ME, Jones PD (August 2003). «Global Surface Temperatures over the Past Two Millennia». Global Surface Temperatures over the Past Two Millennia 30 (15):  pp. 1820. doi:10.1029/2003GL017814. Bibcode2003GeoRL..30oCLM5M. http://www.ngdc.noaa.gov/paleo/pubs/mann2003b/mann2003b.html. 
  6. a b Wahl ER, Ammann CM (November 2007). «Robustness of the Mann, Bradley, Hughes reconstruction of Northern Hemisphere surface temperatures: Examination of criticisms based on the nature and processing of proxy climate evidence» (PDF). Climatic Change 85 (1–2):  pp. 33–69. doi:10.1007/s10584-006-9105-7. http://www.cgd.ucar.edu/ccr/ammann/millennium/refs/Wahl_ClimChange2007.pdf. 
  7. Houghton, 2001 Figure 2.20: Millennial Northern Hemisphere (NH) temperature reconstruction (blue) and instrumental data (red) from AD 1000 to 1999
  8. Houghton, 2001 Figure 2.21: Comparison of warm-season and annual mean multi-proxy-based and warm season tree-ring-based millennial Northern Hemisphere temperature reconstructions
  9. Houghton, 2001 2.3.5 Summary
  10. http://www.ipcc.ch/publications_and_data/ar4/wg1/en/spmsspm-a-palaeoclimatic.html
  11. Powell, Alvin (24 de abril de 2003). «Sun's warming is global: CfA lecture links solar activity and climate change». Harvard University Gazette. Consultado el 17 de abril de 2007.
  12. Bradley RS, Hughes MK, Mann ME (August 2006). «Authors were clear about hockey-stick uncertainties». Nature 442 (7103):  pp. 627. doi:10.1038/442627b. PMID 16900179. Bibcode2006Natur.442..627B. 
  13. Mann ME, Bradley RS, Hughes MK (1999). «Northern hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations» (PDF). Geophys. Res. Lett. 26 (6):  pp. 759–762. doi:10.1029/1999GL900070. Bibcode1999GeoRL..26..759M. https://www.ncdc.noaa.gov/paleo/pubs/millennium-camera.pdf. 
  14. McIntyre S, McKitrick R (2005). «Hockey sticks, principal components, and spurious significance». Geophys. Res. Lett. 32 (3):  pp. L03710. doi:10.1029/2004GL021750. Bibcode2005GeoRL..3203710M. http://www.agu.org/pubs/crossref/2005/2004GL021750.shtml. 
  15. [1]Uso incorrecto de la plantilla enlace roto (enlace roto disponible en Internet Archive; véase el historial y la última versión).
  16. Michael E. Mann, Raymond S. Bradley, Malcolm K. Hughes Note on Paper by McIntyre and McKitrick in "Energy and Environment"
  17. Climate Change 2007: The Physical Science Basis, Ch. 6

Enlaces externos[editar]