A Human Influence On Precipitation ‘Has Yet To Be Detected’
“Climate model output suggests decreasing rainfall as a consequence of anthropogenic greenhouse gas radiative forcing.”
“[I]f anthropogenic forcing has impacted the [regional rainfall pattern], the signal has yet to be detected above the level of natural climate variability.” – Lachniet et al., 2017
According to climate models, precipitation trends were supposed to have intensified as a consequence of human activity.
And yet after compiling decades of observational and proxy (paleoclimate) evidence, it has been determined there has been no detectable global-scale human influence on rainfall patterns in the last hundred years (even hundreds of years). Instead, any variability in the hydrological cycle can be strongly linked to non-anthropogenic forcing mechanisms, namely solar activity and natural oceanic/atmospheric oscillations (NAO, PDO, AMO, ENSO).
The hydrological cycle is expected to intensify in response to global warming. Yet, little unequivocal evidence of such an acceleration has been found on a global scale. This holds in particular for terrestrial evaporation, the crucial return flow of water from land to atmosphere. Here we use satellite observations to reveal that continental evaporation has increased in northern latitudes, at rates consistent with expectations derived from temperature trends. However, at the global scale, the dynamics of the El Niño/Southern Oscillation (ENSO) have dominated the multi-decadal variability.
Modern Precipitation Trends Similar To Past Centuries
Overall, the inter-annual and inter-decadal variability of rainfall and runoff observed in the modern record (Coefficient of Variation (CV) of 22% for rainfall, 42% for runoff) is similar to the variability experienced over the last 500 years (CV of 21% for rainfall and 36% for runoff). However, the modern period is wetter on average than the pre-instrumental (13% higher for rainfall and 23% higher for runoff). Figure 9 also shows that the reconstructions contain a number of individual years (both wet and dry) of greater magnitude than what has been recorded in the instrumental record.
A nested July–June precipitation reconstruction for the period AD 1777–2012 was developed from multi-century tree-ring records of Pinus sylvestris L. (Scots pine) for the Republic of Khakassia in Siberia, Russia. … The longest reconstructed dry period, defined as consecutive years with less than 25th percentile of observed July–June precipitation, was 3 years (1861–1863). There was no significant difference in the number dry and wet periods during the 236 years of the reconstructed precipitation.
Five of the six coupled ocean-atmosphere climate models of the Paleoclimate Modeling Intercomparison Project Phase III (PMIP3), can reproduce the south-north dipole mode of precipitation in eastern China, and its likely link with ENSO. However, there is mismatch in terms of their time development. This is consistent with an important role of the internal variability in the precipitation field changes over the past 500 years.
20th century precipitation variability in southern Tibet falls within the range of natural variability in the last 4100 yr, and does not show a clear trend of increasing precipitation as projected by models. Instead, it appears that poorly understood multidecadal to centennial internal modes of monsoon variability remained influential throughout the last 4100 yr. … Until we have a predictive understanding of multidecade to multi-century variability in the Asian monsoon system, it would be wise to consider the risk of prolonged periods of anomalously dry and wet monsoon conditions to be substantial (Ault et al., 2014). Such variability may also explain why the predicted anthropogenic increase in Asian monsoon precipitation is not widely observed.
Corresponding ~4-8 year periodicities identified from Wavelet analysis of particle size data from Pescadero Marsh in Central Coast California and rainfall data from San Francisco reflect established ENSO periodicity, as further evidenced in the Multivariate ENSO Index (MEI), and thus confirms an important ENSO control on both precipitation and barrier regime variability.
In this study, a monthly water-balance model is used to simulate monthly runoff for 2109 hydrologic units (HUs) in the conterminous United States (CONUS) for water-years 1901 through 2014. … Results indicated that … the variability of precipitation appears to have been the principal climatic factor determining drought, and for most of the CONUS [conterminous US], drought frequency appears to have decreased during the 1901 through 2014 period.
[M]onsoon dynamics appear to be linked to low-frequency variability in the ENSO and NAO, suggesting that ocean-atmosphere processes in the tropical oceans drive rainfall in Mesoamerica. … Climate model output suggests decreasing rainfall as a consequence of anthropogenic greenhouse gas radiative forcing (Rauscher et al., 2008; Saenz-Romero et al., 2010). Our data show, however, that the response of the monsoon will be strongly modulated by the changes in ENSO and the NAO mean states … Our data also show that the magnitude of Mesoamerican monsoon variability over the modern era when the anthropogenic radiative forcing has dominated over solar and volcanic forcings (Schmidt et al., 2012) is within the natural bounds of rainfall variations over the past 2250 years. This observation suggests that if anthropogenic forcing has impacted the Mesoamerican monsoon, the signal has yet to be detected above the level of natural climate variability, and the monsoon response to direct radiative forcing and indirect ocean-atmosphere forcings may yet to be fully realized.
Past, Modern Precipitation Patterns Modulated By Solar Forcing
The precipitation variability on decadal to multi-centurial generally always reflects changes in solar activity and large-scale circulation, e.g., the ENSO and the EASM [East Asian Summer Monsoon] (Chen et al., 2011; Vleeschouwer et al., 2012; Feng et al., 2014). [D]uring the MWP [Medieval Warm Period], the wetter climate in this region was consistent with more frequent ENSO events, stronger EASM and higher solar activity, whereas the opposite was found for the LIA. In particular, d13Cac fluctuations on multi-decadal to centennial scales is consistent with the changes in solar activity, with fewer dry intervals corresponding to periods of minimum solar activity within dating errors, which are referred to as the Oort Minimum (AD 1010-1050), Wolf Minimum (AD 1280-1340), Sporer Minimum (AD 1420-1530), Maunder Minimum (AD 1645-1715) and Dalton Minimum (AD 1795-1820).
Climatic periodicities recorded in lake sediment magnetic susceptibility data: Further evidence for solar forcing on Indian summer monsoon … The results obtained from this study show that solar variations are the main controlling factor of the southwest monsoon.
The frequencies represent the influence of the Pacific Decadal Oscillation (PDO) and solar activity on the precipitation from the southwestern United States. In addition, solar activity has exerted a greater effect than PDO on the precipitation in the southwestern United States over the past 120 years. By comparing the trend of droughts with the two fundamental frequencies, we find that both the droughts in the 1900s and in the 21st century were affected by the PDO and solar activity, whereas the droughts from the 1950s to the 1970s were mainly affected by solar activity.
Decadal resolution record of Oman upwelling indicates solar forcing of the Indian summer monsoon (9–6 ka) … We use geochemical parameters, transfer functions of planktic foraminiferal assemblages and Mg / Ca palaeothermometry, and find evidence corroborating previous studies showing that upwelling intensity varies significantly in coherence with solar sunspot cycles. The dominant ∼ 80–90-year Gleissberg cycle apparently also affected bottom-water oxygen conditions.
The time series of sunspot number and the precipitation in the north-central China (108° ∼ 115° E, 33° ∼ 41° N) over the past 500 years (1470–2002) are investigated, through periodicity analysis, cross wavelet transform and ensemble empirical mode decomposition analysis. The results are as follows: the solar activity periods are determined in the precipitation time series of weak statistical significance, but are found in decomposed components of the series with statistically significance; the Quasi Biennial Oscillation (QBO) is determined to significantly exist in the time series, and its action on precipitation is opposite to the solar activity; the sun is inferred to act on precipitation in two ways, with one lagging the other by half of the solar activity period.
[A]t least six centennial droughts occurred at about 7300, 6300, 5500, 3400, 2500 and 500 cal yr BP. Our findings are generally consistent with other records from the ISM [Indian Summer Monsoon] region, and suggest that the monsoon intensity is primarily controlled by solar irradiance on a centennial time scale.
Abrupt enhancements in the flux of pedogenic magnetite in the stalagmite agree well with the timing of known regional paleofloods and with equatorial El Niño−Southern Oscillation (ENSO) patterns, documenting the occurrence of ENSO-related storms in the Holocene. Spectral power analyses reveal that the storms occur on a significant 500-y cycle, coincident with periodic solar activity and ENSO variance, showing that reinforced (subdued) storms in central China correspond to reduced (increased) solar activity and amplified (damped) ENSO. Thus, the magnetic minerals in speleothem HS4 preserve a record of the cyclic storms controlled by the coupled atmosphere−oceanic circulation driven by solar activity.
Western Mediterranean Holocene record of abrupt hydro-climatic changes … Imprints of North Atlantic meltwater discharges, NAO and solar forcing …Early Holocene winter rain minima are in phase with cooling events and millennial-scale meltwater discharges in the sub-polar North Atlantic. … [A] significant hydro-climatic shift at the end of the African Humid Period (∼5 ka) indicates a change in climate forcing mechanisms. The Late Holocene climate variability in the Middle Atlas features a multi-centennial-scale NAO-type pattern, with Atlantic cooling and Western Mediterranean winter rain maxima generally associated with solar minima.
An increase in atmospheric moisture for the warm period of the year (May–September) since 1890s, and mean annual temperatures since the 1950s was identified. During the same time period, there was a marked increase in amplitude of the annual variations for temperature and precipitation. … These fluctuations [atmospheric moisture, mean annual temperatures] are consistent with 10–12-years Schwabe–Wolf, 22-years Hale, and the 32–36-years Bruckner Solar Cycles. There was an additional relationship found between high-frequency (short-period) climate fluctuations, lasting for about three years, and 70–90-years fluctuations of the moisture regime in the study region corresponding to longer cycles.
The 11 yr solar cycle, also known as Schwabe cycle, represents the smallest-scaled solar cyclicity and is traced back to sunspot activity (Douglass, 1928; Lean, 2000), which has a measurable effect on the Earth’s climate, as indicated by the Maunder minimum (Usoskin et al., 2015). Global climate feedback reactions to solar irradiance variations caused by sunspots are complex and hypothesized to be triggered by (1) variation in total energy input (Cubasch and Voss, 2000), (2) the influence of ultraviolet light intensity variation on composition of the stratosphere (Lean and Rind, 2001), (3) the effect of cosmic rays on cloud formation (Marsh and Svensmark, 2000; Sun and Bradley, 2002), and/or (4) the effect of high-energy particles on the strato- and mesosphere (Jackman et al., 2005). … [L]ike today, sunspot activity caused fluctuations of cosmic radiation input to the atmosphere, affecting cloud formation and annual rates of precipitation.
[S]olar activity drove Holocene variations in both East Asian Monsoon (EAM) and El Niño Southern Oscillation (ENSO).
Our results imply that the synchronous change in the Asian–Australian monsoon may be caused by inherent solar variations, further strengthening previous findings.
Past, Modern Precipitation Patterns Modulated By AMO/PDO/NAO/ENSO
Statistically significant relationships between the British flood index, the Atlantic Meridional Oscillation and the North Atlantic Oscillation Index are identified. The use of historical records identifies that the largest floods often transcend single catchments affecting regions and that the current flood-rich period is not unprecedented. … Solar forcing can manifest itself in a variety of different ways on flood patterns through modification of the climate (Benito et al., 2004). Several series indicated increased flood frequency during the late eighteenth century corresponding to the Dalton Minimum (AD 1790–1830), with notable flooding across catchments in the 8-year period AD 1769 1779, which was a climatic period considered to include the sharpest phases of temperature variability during the “Little Ice Age” (Lamb, 1995; Wanner et al., 2008).
[W]e investigate the impact of internal climate variability and external climate forcings on ISMR on decadal to multi-decadal timescales over the past 400 years. The results show that AMO, PDO, and Total Solar Irradiance (TSI) play a considerable role in controlling the wet and dry decades of ISMR [Indian summer monsoon rainfall]. Resembling observational findings most of the dry decades of ISMR occur during a negative phase of AMO and a simultaneous positive phase of PDO.
This study analyzes these low-frequency patterns of precipitation in Chile (>30 years), and their relationship to global Sea Surface Temperatures (SSTs), with special focus on associations with the Pacific Decadal Oscillation (PDO) and the Atlantic Multi-decadal Oscillation (AMO) indices. … We conclude that a significant multi-decadal precipitation cycle between 40 and 60 years is evident at the rain gauges located in the subtropical and extratropical regions of Chile. This low-frequency variability seems to be largely linked to PDO and AMO modulation.
We document that long-term patterns in temperature and precipitation are recorded in dripwater patterns of Bunker Cave and that these are linked to the North Atlantic Oscillation (NAO).
This paper investigates an annually-laminated (varved) record from the western Canadian Arctic and finds that the varves are negatively correlated with both the instrumental Pacific Decadal Oscillation (PDO) during the past century and also with reconstructed PDO over the past 700 years, suggesting drier Arctic conditions during high-PDO phases, and vice versa. These results are in agreement with known regional teleconnections, whereby the PDO is negatively and positively correlated with summer precipitation and mean sea level pressure respectively.
Our study demonstrated that flood frequency and climate changes at centennial-to-millennial time scales in South Korea have been coupled mainly with ENSO activity, suggesting that the hydrologic changes, including flooding and drought, in East Asia are coupled to the centennial-to-millennial-scale atmospheric-oceanic circulation changes represented by the ENSO pattern.
Evidence derived from instrumental observations suggest that Atlantic variability, associated with changes in SSTs and fluctuations in the strength of the Atlantic Meridional Overturning Circulation (AMOC), is directly linked with broader scale climate variability, including Brazilian and Sahel precipitation (Folland et al., 1986 and Folland et al., 2001), Atlantic hurricanes and storm tracks (Goldenberg et al., 2001 and Emanuel, 2005), and North American and European temperatures (Sutton and Hodson, 2005, Knight et al., 2006 and Mann et al., 2009).
According to our results, the central Mexican climate has been predominantly controlled by the combined influence of the 20-year Pacific Decadal Oscillation (PDO) and the 70-year Atlantic Multidecadal Oscillation (AMO).
Seven periods of increased water level, varying in duration, occurred during the backbarrier period, with El Niño-Southern Oscillation (ENSO) likely the main climatic mechanism causing these periodic shifts in the paleo-precipitation levels. We suggest that the deepest water levels detected over the last ~3200 years correlate with periods of increased ENSO activity.
This article appeared on the NoTricksZone website at http://notrickszone.com/2017/10/02/28-new-papers-solar-ocean-cycles-modulate-rainfall-trends/