Influencia del Sol en el sistema climático terrestre

Uno de los grandes misterios sin resolver por la ciencia del clima es la cuestión sobre en qué medida las variaciones de la actividad solar son responsables de los cambios climáticos. Una gran cantidad de investigaciones geo-paleoclimatológicas no dejan lugar a dudas sobre la existente influencia solar sobre el clima. Sin embargo, para explicar tal influencia se requiere de un mecanismo amplificador de la energía solar, ya que las fluctuaciones solares en el intervalo de radiación de luz visible son probablemente demasiado bajas para producir por si solas los efectos observados. Hasta ahora se han postulado dos candidatos como los más probables. Por un lado tendríamos el llamado efecto Svensmark, según el cual la radiación solar podría provocar la génesis de nubes mediante un aumento en los procesos de nucleación. Por otra parte podemos considerar la radiación UV, mucho más variable en amplitud que la porción de luz visible del sol. La radiación UV genera en la estratosfera el ozono. Van apareciendo cada vez más indicios de que los procesos observados en la estratosfera también se produzcan en capas más de la atmósfera, en las que se desarrollan los patrones climáticos. A continuación queremos emprender un viaje a través de la literatura reciente. ¿Qué novedades hay sobre los sistemas de amplificación del efecto solar estratosférico?

En noviembre de 2012, un grupo de investigadores entorno a David Thompson nos dió una gran sorpresa en la revista Nature: presentaban un nuevo conjunto de datos de la temperatura de la estratosfera media y alta que difería significativamente de las curvas de temperatura que se habían tenido como ciertas anteriormente. Ha sido una pequeña revolución en ese campo. Al parecer, los modelos al uso no servían para nada. Los modelos han de ser revisados para adaptarse a la nueva curva de datos. Esto es particularmente interesante, porque en esas zonas de la atmósfera las emisiones radiativas del CO2 y la concentración de los gases que agotan el ozono juegan un papel fundamental. Aquí el resumen de la obra:

The mystery of recent stratospheric temperature trends
A new data set of middle- and upper-stratospheric temperatures based on reprocessing of satellite radiances provides a view of stratospheric climate change during the period 1979–2005 that is strikingly different from that provided by earlier data sets. The new data call into question our understanding of observed stratospheric temperature trends and our ability to test simulations of the stratospheric response to emissions of greenhouse gases and ozone-depleting substances. Here we highlight the important issues raised by the new data and suggest how the climate science community can resolve them.

También Katja Matthes y sus colegas del Instituto Geomar de Kiel trabajan en la exploración del amplificador solar estratosférico. Este equipo ha publicado en los últimos años tres obras que me gustaría presentar aquí. En septiembre de 2012 apareció en el Journal of Geophysical Research un primer paper dirigido por Christof Petrick. En el estudio se trata de variaciones de las UV solares que causan cambios estratosféricos que se manifiestan y desarrollan a través de los ciclos oceánicos. El grupo de Matthes llama a este mecanismo  «top-down mechanism». Aquí el resumen:

Impact of the solar cycle and the QBO on the atmosphere and the ocean
The Solar Cycle and the Quasi-Biennial Oscillation are two major components of natural climate variability. Their direct and indirect influences in the stratosphere and troposphere are subject of a number of studies. The so-called “top-down’ mechanism describes how solar UV changes can lead to a significant enhancement of the small initial signal and corresponding changes in stratospheric dynamics. How the signal then propagates to the surface is still under investigation. We continue the “top-down’ analysis further down to the ocean and show the dynamical ocean response with respect to the solar cycle and the QBO. For this we use two 110-year chemistry climate model experiments from NCAR’s Whole Atmosphere Community Climate Model (WACCM), one with a time varying solar cycle only and one with an additionally nudged QBO, to force an ocean general circulation model, GFZ’s Ocean Model for Circulation and Tides (OMCT). We find a significant ocean response to the solar cycle only in combination with a prescribed QBO. Especially in the Southern Hemisphere we find the tendency to positive Southern Annular Mode (SAM) like pattern in the surface pressure and associated wind anomalies during solar maximum conditions. These atmospheric anomalies propagate into the ocean and induce deviations in ocean currents down into deeper layers, inducing an integrated sea surface height signal. Finally, limitations of this study are discussed and it is concluded that comprehensive climate model studies require a middle atmosphere as well as a coupled ocean to investigate and understand natural climate variability.

En abril de 2013, el grupo Matthes con Ermolli et al, publicaba en la revista Atmospheric Chemistry and Physics otro estudio sobre el tema. El mensaje principal: todos los modelos climáticos existentes subestimaron  las variaciones UV en un factor de 6.4. Es un factor de enorme magnitud. En realidad, las variaciones UV eran, por tanto, en promedio 5 veces más grandes de lo que se presumía. Ermolli y sus colegas predicen que los efectos atmosféricos de las variaciones en la radiación UV son, en consecuencia, mucho mayores que lo que los modelos presuponían anteriormente. Aquí el resumen:

Recent variability of the solar spectral irradiance and its impact on climate modelling
The lack of long and reliable time series of solar spectral irradiance (SSI) measurements makes an accurate quantification of solar contributions to recent climate change difficult. Whereas earlier SSI observations and models provided a qualitatively consistent picture of the SSI variability, recent measurements by the SORCE (SOlar Radiation and Climate Experiment) satellite suggest a significantly stronger variability in the ultraviolet (UV) spectral range and changes in the visible and near-infrared (NIR) bands in anti-phase with the solar cycle. A number of recent chemistry-climate model (CCM) simulations have shown that this might have significant implications on the Earth’s atmosphere. Motivated by these results, we summarize here our current knowledge of SSI variability and its impact on Earth’s climate.
We present a detailed overview of existing SSI measurements and provide thorough comparison of models available to date. SSI changes influence the Earth’s atmosphere, both directly, through changes in shortwave (SW) heating and therefore, temperature and ozone distributions in the stratosphere, and indirectly, through dynamical feedbacks.We investigate these direct and indirect effects using several state-of-the art CCM simulations forced with measured and modelled SSI changes. A unique asset of this study is the use of a common comprehensive approach for an issue that is usually addressed separately by different communities.
We show that the SORCE measurements are difficult to reconcile with earlier observations and with SSI models. Of the five SSI models discussed here, specifically NRLSSI (Naval Research Laboratory Solar Spectral Irradiance), SATIRE-S (Spectral And Total Irradiance REconstructions for the Satellite era), COSI (COde for Solar Irradiance), SRPM (Solar Radiation Physical Modelling), and OAR (Osservatorio Astronomico di Roma), only one shows a behaviour of the UV and visible irradiance qualitatively resembling that of the recent SORCE measurements. However, the integral of the SSI computed with this model over the entire spectral range does not reproduce the measured cyclical changes of the total solar irradiance, which is an essential requisite for realistic evaluations of solar effects on the Earth’s climate in CCMs.
We show that within the range provided by the recent SSI observations and semi-empirical models discussed here, the NRLSSI model and SORCE observations represent the lower and upper limits in the magnitude of the SSI solar cycle variation.
The results of the CCM simulations, forced with the SSI solar cycle variations estimated from the NRLSSI model and from SORCE measurements, show that the direct solar response in the stratosphere is larger for the SORCE than for the NRLSSI data. Correspondingly, larger UV forcing also leads to a larger surface response.
Finally, we discuss the reliability of the available data and we propose additional coordinated work, first to build composite SSI data sets out of scattered observations and to refine current SSI models, and second, to run coordinated CCM experiments.

En septiembre de 2015 se publicaba el trabajo de Matthes, Thiéblemont et al., en Nature Communications. Según este trabajo, la Oscilación del Atlántico Norte presenta una vinculación a la actividad solar con un retraso de 1-2 años. Los investigadores utilizaron un modelo climático que modela la atmósfera hasta una altitud de 140 kilometros y por lo tanto tiene en cuenta el efecto de la radiación ultravioleta sobre la química de la estratosfera, sobre la formación de ozono, por ejemplo. Aquí el resumen:

Solar forcing synchronizes decadal North Atlantic climate variability
Quasi-decadal variability in solar irradiance has been suggested to exert a substantial effect on Earth’s regional climate. In the North Atlantic sector, the 11-year solar signal has been proposed to project onto a pattern resembling the North Atlantic Oscillation (NAO), with a lag of a few years due to ocean-atmosphere interactions. The solar/NAO relationship is, however, highly misrepresented in climate model simulations with realistic observed forcings. In addition, its detection is particularly complicated since NAO quasi-decadal fluctuations can be intrinsically generated by the coupled ocean-atmosphere system. Here we compare two multi-decadal ocean-atmosphere chemistry-climate simulations with and without solar forcing variability. While the experiment including solar variability simulates a 1–2-year lagged solar/NAO relationship, comparison of both experiments suggests that the 11-year solar cycle synchronizes quasi-decadal NAO variability intrinsic to the model. The synchronization is consistent with the downward propagation of the solar signal from the stratosphere to the surface.

Usando un modelo climático que tiene en cuenta la estratosfera, un equipo de investigación dirigido por Lon Hood fue capaz de demostrar un impacto del ciclo solar de 11 años en el desarrollo de la temperatura invernal del Pacífico. El trabajo fue publicado en octubre de 2013, por Journal of Climate. Aquí el resumen:

The Surface Climate Response to 11-Yr Solar Forcing during Northern Winter: Observational Analyses and Comparisons with GCM Simulations
The surface climate response to 11-yr solar forcing during northern winter is first reestimated by applying a multiple linear regression (MLR) statistical model to Hadley Centre sea level pressure (SLP) and sea surface temperature (SST) data over the1880–2009 period. In addition to a significant positive SLP response in the North Pacific found in previous studies, a positive SST response is obtained across the midlatitude North Pacific. Negative but insignificant SLP responses are obtained in the Arctic. The derived SLP response at zero lag therefore resembles a positive phase of the Arctic Oscillation (AO). Evaluation of the SLP and SST responses as a function of phase lag indicates that the response evolves from a negative AO-like mode a few years before solar maximum to a positive AO-like mode at and following solar maximum. For comparison, a similar MLR analysis is applied to model SLP and SST data from a series of simulations using an atmosphere–ocean general circulation model with a well-resolved stratosphere. The simulations differed only in the assumed solar cycle variation ofstratospheric ozone. It is found that the simulation that assumed an ozone variation estimated from satellite data produces solar SLP and SST responses that are most consistent with the observational results, especially during a selected centennial period. In particular, a positive SLP response anomaly is obtained in the northeastern Pacific and a corresponding positive SST response anomaly extends across the midlatitude North Pacific. The model response versus phase lag also evolves from a mainly negative AO-like response before solar maximum to a mainly positive AO response at and following solar maximum.

En una línea similar ya había aparecido en el Journal of Geophysical Research el trabajo de Chiodo et al., Que en marzo de 2012. Ya entonces los autores habían encontrado un efecto del ciclo solar de 11 años sobre el sistema climático en la atmósfera. A continuación el resumen:

The 11 year solar cycle signal in transient simulations from the Whole Atmosphere Community Climate Model
The atmospheric response to the 11 year solar cycle (SC) and its combination with the quasi-biennal oscillation (QBO) are analyzed in four simulations of the Whole Atmosphere Community Climate Model version 3.5 (WACCM3.5), which were performed with observed sea surface temperatures, volcanic eruptions, greenhouse gases, and a nudged QBO. The analysis focuses on the annual mean response of the model to the SC and on the evolution of the solar signal during the Northern Hemispheric winter. WACCM3.5 simulates a significantly warmer stratosphere under solar maximum conditions compared to solar minimum. The vertical structure of the signal in temperature and ozone at low latitudes agrees with observations better than previous versions of the model. The temperature and wind response in the extratropics is more uncertain because of its seasonal dependence and the large dynamical variability of the polar vortex. However, all four simulations reproduce the observed downward propagation of zonal wind anomalies from the upper stratosphere to the lower stratosphere during boreal winter resulting from solar-induced modulation of the polar night jet and the Brewer-Dobson circulation. Combined QBO-SC effects in the extratropics are consistent with observations, but they are not robust across the ensemble members. During boreal winter, solar signals are also found in tropospheric circulation and surface temperature. Overall, these results confirm the plausibility of proposed dynamical mechanisms driving the atmospheric response to the SC. The improvement of the model climatology and variability in the polar stratosphere is the basis for the success in simulating the evolution and magnitude of the solar signal.

También muy debatido, pero no rebatido, es el trabajo del grupo liderado por Michael Beckmann, quien presentó su publicación en abril 2014  en Methods in Ecology and Evolution. Beckmann y sus colegas reúnen un conjunto de datos UV-B y encontraron una buena correlación entre la temperatura media global y la intensidad de la radiación UV-B. Aquí está la versión corta:

glUV: a global UV-B radiation data set for macroecological studies

1. Macroecology has prospered in recent years due in part to the wide array of climatic data, such as those provided by the WorldClim and CliMond data sets, which has become available for research. However, important environmental variables have still been missing, includingspatial data sets on UV-B radiation, an increasingly recognized driver of ecological processes.

2. We developed a set of global UV-B surfaces (glUV) suitable to match common spatial scales in macroecology. Our data set is based on remotely sensed records from NASA’s Ozone Monitoring Instrument (Aura-OMI). Following a similar approach as for the WorldClim and CliMond data sets, we processed daily UV-B measurements acquired over a period of eight years into monthly mean UV-B data and six ecologically meaningful UV-B variables with a 15-arc minute resolution. These bioclimatic variables represent Annual Mean UV-B, UV-B Seasonality, Mean UV-B of Highest Month, Mean UV-B of Lowest Month, Sum of Monthly Mean UV-B during Highest Quarter and Sum of Monthly Mean UV-B during Lowest Quarter. We correlated our data sets with selected variables of existing bioclimatic surfaces for land and with Terra–MODIS Sea Surface Temperature for ocean regions to test for relations to known gradients and patterns.

3. UV-B surfaces showed a distinct seasonal variance at a global scale, while the intensity of UV-B radiation decreased towards higher latitudes and was modified by topographic and climatic heterogeneity. UV-B surfaces were correlated with global mean temperature and annual mean radiation data, but exhibited variable spatial associations across the globe. UV-B surfaces were otherwise widely independent of existing bioclimatic surfaces.

4. Our data set provides new climatological information relevant for macroecological analyses. As UV-B is a known driver of numerous biological patterns and processes, our data set offers the potential to generate a better understanding of these dynamics in macroecology, biogeography, global change research and beyond. The glUV data set containing monthly mean UV-B data and six derived UV-B surfaces is freely available for download at: http://www.ufz.de/gluv.

Realmente sorprendentes fueron los resultados obtenidos por un grupo de investigadores entorno a  Tao Li, publicados en junio de 2013 en Geophysical Research Letters. Los científicos descubrieron que el fenómeno de El Niño es apreciable hasta en las más altas capas de la atmósfera. Li y sus colegas lograron probar los efectos de El Niño hasta la mesosfera, que se extiende en una altura de 50 a 85 km por encima de la estratosfera. Se trata de una prueba más de la significativa relación climática entre las distintas capas de la atmósfera; relaciones que apenas se consideran en los modelos climáticos del IPCC. Aquí el resumen:

Influence of El Niño-Southern Oscillation in the mesosphere
Using the middle atmosphere temperature data set observed by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) satellite experiment between 2002 and 2012, and temperatures simulated by the Whole Atmospheric Community Climate Model version 3.5 (WACCM3.5) between 1953 and 2005, we studied the influence of El Niño-Southern Oscillation (ENSO) on middle atmosphere temperature during the Northern Hemisphere (NH) wintertime. For the first time, a significant winter temperature response to ENSO in the middle mesosphere has been observed, with an anomalous warming of ~1.0 K/MEI (Multivariate ENSO Index) in the tropics and an anomalous cooling of ~ −2.0 K/MEI in the NH middle latitudes. The observed temperature responses to ENSO in the mesosphere are opposite to those in the stratosphere, in agreement with previous modeling studies. Temperature responses to ENSO observed by SABER show similar patterns to those simulated by the WACCM3.5 model. Analysis of the WACCM3.5 residual mean meridional circulation response to ENSO reveals a significant downwelling in the tropical mesosphere and upwelling in the NH middle and high latitudes during warm ENSO events, which is mostly driven by anomalous eastward gravity wave forcing in the NH mesosphere.

Otro interesante estudio sobre el tema viene de la mano de Maycock et al., publicado en junio de 2015 en el  Journal of Geophysical Research. Los autores modelaron un gran mínimo solar, tal y como predicen la mayoría de los físicos solares para las próximas décadas. Calcularon un enfriamiento de la capa límite de la estratosfera superior de 1,2 ° C. Dadas las limitaciones de los modelos usados, los investigadores apenas pudieron detectar un efecto global sobre la temperatura a nivel atmosférico bajo. Sin embargo, son capaces de apreciar en ciertas regiones del mundo cambios climáticos relacikonados con la inactividad solar. Aquí el resumen:

Possible impacts of a future grand solar minimum on climate: Stratospheric and global circulation changes
It has been suggested that the Sun may evolve into a period of lower activity over the 21st century. This study examines the potential climate impacts of the onset of an extreme “Maunder Minimum-like” grand solar minimum using a comprehensive global climate model. Over the second half of the 21st century, the scenario assumes a decrease in total solar irradiance of 0.12% compared to a reference Representative Concentration Pathway 8.5 experiment. The decrease in solar irradiance cools the stratopause (∼1 hPa) in the annual and global mean by 1.2 K. The impact on global mean near-surface temperature is small (∼−0.1 K), but larger changes in regional climate occur during the stratospheric dynamically active seasons. In Northern Hemisphere wintertime, there is a weakening of the stratospheric westerly jet by up to∼3–4 m s⁻¹, with the largest changes occurring in January–February. This is accompanied by a deepening of the Aleutian Low at the surface and an increase in blocking over Northern Europe and the North Pacific. There is also an equatorward shift in the Southern Hemisphere midlatitude eddy-driven jet in austral spring. The occurrence of an amplified regional response during winter and spring suggests a contribution from a top-down pathway for solar-climate coupling; this is tested using an experiment in which ultraviolet (200–320 nm) radiation is decreased in isolation of other changes. The results show that a large decline in solar activity over the 21st century could have important impacts on the stratosphere and regional surface climate.

Otra obra que muestra el vínculo estrecho entre la estratosfera y la troposfera es la del grupo liderado por Tao Mengchu que publicó sus trabajos en junio de 2015, en Geophysical Research Letters. En sus «Key Points», los autores explican que habían encontrado un impacto en el contenido de vapor de agua tropical como resultado de eventos de calentamiento estratosférico („Tropical water vapor response to stratospheric major warming“). Aquí el resumen:

Impact of stratospheric major warmings and the quasi-biennial oscillation on the variability of stratospheric water vapor
Based on simulations with the Chemical Lagrangian Model of the Stratosphere for the 1979–2013 period, driven by the European Centre for Medium-Range Weather Forecasts ERA-Interim reanalysis, we analyze the impact of the quasi-biennial oscillation (QBO) and of Major Stratospheric Warmings (MWs) on the amount of water vapor entering the stratosphere during boreal winter. The amplitude of H₂O variation related to the QBO amounts to 0.5 ppmv. The additional effect of MWs reaches its maximum about 2–4 weeks after the central date of the MW and strongly depends on the QBO phase. Whereas during the easterly QBO phase there is a pronounced drying of about 0.3 ppmv about 3 weeks after the MW, the impact of the MW during the westerly QBO phase is smaller (about 0.2 ppmv) and more diffusely spread over time. We suggest that the MW-associated enhanced dehydration combined with a higher frequency of MWs after the year 2000 may have contributed to the lower stratospheric water vapor after 2000.

También es interesante una publicación de octubre de 2014 en Nature Communications, firmada por Andersson et al. Ya el título despierta la curiosidad. Se trata de la intensamente buscada conexión climática atmosférica entre el Sol y la Tierra, a través del ozono como amplificador en la mesosfera. La «precipitación energética de electrones » (EEP) mencionada en el artículo está directamente influenciada por la actividad solar. Los autores describen un proceso que sugiere que la EEP juega un papel importante a la hora de explicar la influencia solar sobre el clima de la Tierra. Aquí el resumen:

Missing driver in the Sun–Earth connection from energetic electron precipitation impacts mesospheric ozone
Energetic electron precipitation (EEP) from the Earth’s outer radiation belt continuously affects the chemical composition of the polar mesosphere. EEP can contribute to catalytic ozone loss in the mesosphere through ionization and enhanced production of odd hydrogen. However, the long-term mesospheric ozone variability caused by EEP has not been quantified or confirmed to date. Here we show, using observations from three different satellite instruments, that EEP events strongly affect ozone at 60–80 km, leading to extremely large (up to 90%) short-term ozone depletion. This impact is comparable to that of large, but much less frequent, solar proton events. On solar cycle timescales, we find that EEP causes ozone variations of up to 34% at 70–80 km. With such a magnitude, it is reasonable to suspect that EEP could be an important part of solar influence on the atmosphere and climate system.

Y, por último: los efectos de la actividad solar  sobre la temperatura en la parte más alta de la atmósfera, la termosfera, han quedado descritos por un grupo liderado por Martin Młyńczak en abril de 2014 en Geophysical Research Letters. Las temperaturas varían al ritmo del ciclo solar de 11 años, asociado a cambios en la concentración de NO y CO2. Aquí el resumen:

Influence of solar variability on the infrared radiative cooling of the thermosphere from 2002 to 2014
Infrared radiative cooling of the thermosphere by carbon dioxide (CO₂, 15 µm) and by nitric oxide (NO, 5.3 µm) has been observed for 12 years by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics satellite. For the first time we present a record of the two most important thermospheric infrared cooling agents over a complete solar cycle. SABER has documented dramatic variability in the radiative cooling on time scales ranging from days to the 11 year solar cycle. Deep minima in global mean vertical profiles of radiative cooling are observed in 2008–2009. Current solar maximum conditions, evidenced in the rates of radiative cooling, are substantially weaker than prior maximum conditions in 2002–2003. The observed changes in thermospheric cooling correlate well with changes in solar ultraviolet irradiance and geomagnetic activity during the prior maximum conditions. NO and CO₂ combine to emit 7 × 10¹⁸ more Joules annually at solar maximum than at solar minimum.

No seré yo quien les diga ahora que el cambio climático es exclusivamente debido a la actividad solar. Pero de lo que no me cabe la menor duda es del papel relevante del Sol en nuestro clima. Papel que, como queda patente en muchos de los artículos que les he traído, no se había tenido en cuenta para realizar los modelos climáticos sobre los que se basa la teoría de Calentamiento Global Antropogénico por emisiones de CO2.

¿Sigue pensando que la ciencia es consensuable? ¿Sigue creyendo que ya lo sabemos todo sobre el clima? Cuéntenos lo que opina en comentarios y, si le ha gustado lo leído, no dude en difundirlo. Gracias