Surface thermal perturbations of the recent past at low latitudes – inferences based on borehole temperature data from Eastern Brazil

Abstract

Abstract. Borehole temperature data from the eastern parts of Brazil has been examined in an attempt to extract information on surface thermal perturbations of the recent past at low latitudes. Forward models were employed in the analysis of temperature logs from 17 localities and, in addition, Bayesian inverse modeling was carried out for data from 14 selected sites. The model results have allowed determination of the magnitude as well as the duration of ground surface temperature (GST) changes in three major geographic zones of Brazil. Prominent among such changes are the warming episodes that occurred over much of the subtropical highland regions in the southeastern parts of Brazil. The present magnitude of GST changes in this region are in the range of 2 to 3.5 • C but have had their beginning during the early decades of the 20th century. Nearly similar trends are also seen in temperature-depth profiles of bore holes in the subtropical humid zones of the interior and coastal areas of southern Brazil. The data from semi arid zones of northeast Brazil also indicate occurrence of surface warming events but the magnitudes are in the range of 1.4 to 2.2 • C while the duration of the warming event is larger, extending back into the last decades of the 19th century. There are indications that changes in both climate and vegetation cover contribute to variations in GST. Thus the magnitudes of GST variations are relatively large in localities which have undergone changes in vegetation cover. Also there are indications that GST changes are practically insignificant in areas of tropical rain forest. Another important result emerging from model studies is that the climate was relatively cooler during the 17th and 18th centuries. The climate histories, deduced from geothermal data, are found to be consistent with results of available meteorological records in southern Brazil. Comparative studies also indicate that the magnitudes and duration of recent climate changes in southern and eastern Brazil are similar to those found in other continental areas such as North America, Asia and Europe.Correspondence to: V. M. Hamza (hamza@on.br)

Impact of maximum borehole depths on inverted temperature histories in borehole paleoclimatology

“…For instance, some studies have used boreholes as shallow as 100-150 m (e.g. Majorowicz et al, 1999;Majorowicz and Safanda, 2001;Majorowicz et al, 2002;Hamza et al, 2007), while the many global analyses typically set the minimum depth criterion at 200 m thus employing boreholes 200 m or deeper Harris and Chapman, 2001;Beltrami, 2002;Beltrami and Bourlon, 2004;Pollack and Smerdon, 2004). Our calculations suggest that these depths are likely too shallow, but several caveats are necessary and discussed below.…”

“…These biases consequently impact the estimated magnitude and temporal characteristics of recovered GST histories. Large-scale analyses that involve comparisons of results from deep and very shallow borehole temperature logs (<200 m) (Majorowicz et al, 1999;Majorowicz and Safanda, 2001;Majorowicz et al, 2002;Hamza et al, 2007), yield results that are difficult to evaluate because of potential biases due to preferential depth ranges within regions. This potential bias does not disappear using simultaneous inversion since, as shown in Beltrami et al (1997), the resulting GST histories are dominated by the deepest temperature log or those with larger sampling rates.…”

Climate Changes of the Recent Past in the South American Continent: Inferences Based on Analysis of Borehole Temperature Profiles

“…4. History of ground surface temperature variations associated with climate changes of the recent past, deduced on the basis of functional space inversion models fits to borehole temperature profiles, for selected localities in South America (Adapted with modifications from Hamza et al, 2007).…”

“…Data acquired at shallow depths of less than 20 meters were excluded from analyses for climate changes, avoiding thereby eventual perturbing effects of diurnal and seasonal variations in the reconstruction of surface temperature history. In an earlier study Hamza et al (2007) reported results of geothermal measurements for a number of sites in the Brazilian territory. In the present we have included results of additional studies carried out during the period of 2006 to 2011.…”

New insights into the reconstructed temperature in Portugal over the last 400 years

“…The corresponding root-mean squared error (RMSE) of each estimated linear model is always < 0.01 • C (R-square adjusted > 99.9 %), which means that the errors in the estimation of the geothermal gradients have only minor impacts on the subsequent temperature-depth anomalies. The low borehole depths require a word of caution, as some authors have indicated that a depth of 200 m may be too shallow for climate change assessments (Beltrami et al, 2011;Hamza et al, 2007;Majorowicz et al, 1999). Indeed, the Global Database of Borehole Temperatures and Climate Reconstruc- Since no other geothermal-paleoclimatological observatory is available in Portugal, the conclusions derived from these borehole profiles may be provisional.…”