The development of a 2° lat × 3° long grid of summer drought reconstructions for the continental United States estimated from a dense network of annual tree-ring chronologies is described. The drought metric used is the Palmer Drought Severity Index (PDSI). The number of grid points is 154 and the reconstructions cover the common period 1700–1978. In producing this grid, an automated gridpoint regression method called “point-by-point regression” was developed and tested. In so doing, a near-optimal global solution was found for its implementation. The reconstructions have been thoroughly tested for validity using PDSI data not used in regression modeling. In general, most of the gridpoint estimates of drought pass the verification tests used. In addition, the spatial features of drought in the United States have been faithfully recorded in the reconstructions even though the method of reconstruction is not explicitly spatial in its design. The drought reconstructions show that the 1930s “Dust Bow...

https://journals.ametsoc.org/doi/pdf/10.1175/1520-0442(1999)012%3C1145%3ADRFTCU%3E2.0.CO%3B2

Drought Reconstructions for the Continental United States

[1] We review evidence for climate change over the past several millennia from instrumental and high-resolution climate “proxy” data sources and climate modeling studies. We focus on changes over the past 1 to 2 millennia. We assess reconstructions and modeling studies analyzing a number of different climate fields, including atmospheric circulation diagnostics, precipitation, and drought. We devote particular attention to proxy-based reconstructions of temperature patterns in past centuries, which place recent large-scale warming in an appropriate longer-term context. Our assessment affirms the conclusion that late 20th century warmth is unprecedented at hemispheric and, likely, global scales. There is more tentative evidence that particular modes of climate variability, such as the El Nino/Southern Oscillation and the North Atlantic Oscillation, may have exhibited late 20th century behavior that is anomalous in a long-term context. Regional conclusions, particularly for the Southern Hemisphere and parts of the tropics where high-resolution proxy data are sparse, are more circumspect. The dramatic differences between regional and hemispheric/global past trends, and the distinction between changes in surface temperature and precipitation/drought fields, underscore the limited utility in the use of terms such as the “Little Ice Age” and “Medieval Warm Period” for describing past climate epochs during the last millennium. Comparison of empirical evidence with proxy-based reconstructions demonstrates that natural factors appear to explain relatively well the major surface temperature changes of the past millennium through the 19th century (including hemispheric means and some spatial patterns). Only anthropogenic forcing of climate, however, can explain the recent anomalous warming in the late 20th century.

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2003RG000143

Climate over past millennia

North American drought: Reconstructions, causes, and consequences

This review of late-Holocene palaeoclimatology represents the results from a PAGES/CLIVAR Intersection Panel meeting that took place in June 2006. The review is in three parts: the principal high-resolution proxy disciplines (trees, corals, ice cores and documentary evidence), emphasizing current issues in their use for climate reconstruction; the various approaches that have been adopted to combine multiple climate proxy records to provide estimates of past annual-to-decadal timescale Northern Hemisphere surface temperatures and other climate variables, such as large-scale circulation indices; and the forcing histories used in climate model simulations of the past millennium. We discuss the need to develop a framework through which current and new approaches to interpreting these proxy data may be rigorously assessed using pseudo-proxies derived from climate model runs, where the `answer' is known. The article concludes with a list of recommendations. First, more raw proxy data are required from the diverse disciplines and from more locations, as well as replication, for all proxy sources, of the basic raw measurements to improve absolute dating, and to better distinguish the proxy climate signal from noise. Second, more effort is required to improve the understanding of what individual proxies respond to, supported by more site measurements and process studies. These activities should also be mindful of the correlation structure of instrumental data, indicating which adjacent proxy records ought to be in agreement and which not. Third, large-scale climate reconstructions should be attempted using a wide variety of techniques, emphasizing those for which quantified errors can be estimated at specified timescales. Fourth, a greater use of climate model simulations is needed to guide the choice of reconstruction techniques (the pseudo-proxy concept) and possibly help determine where, given limited resources, future sampling should be concentrated.

/pdf/high-resolution-palaeoclimatology-of-the-last-millennium-a-futl99a0xa.pdf

High-resolution palaeoclimatology of the last millennium: a review of current status and future prospects:

Tree-ring data have been used to reconstruct the mean summer (April-August) temperature of northern Fennoscandia for each year from AD 500 to the present. Summer temperatures have fluctuated markedly on annual, decadal and century timescales. There is little evidence for the existence of a Medieval Warm Epoch, and the Little Ice Age seems to be confined to the relatively short period between 1570 and 1650. This challenges the popular idea that these events were the major climate excursions of the first millennium, occurring synchronously throughout Europe in all seasons. An analysis of past warming trends suggests that any summer warming induced by greenhouse gases may not be detectable in this region until after 2030.

A 1,400-year tree-ring record of summer temperatures in Fennoscandia

Estimates of mean July-August temperatures for northern Fennoscandinavia are made back to 1700 using ring width and maximum latewood density chronologies of Pinus sylvestris L. (Scots pine) as predictors. Several prediction models are used and the best results are achieved with a simple two-variable model where climate in year t is estimated as a function of tree growth in years t and t+ 1 at each site. The best reconstruction equation accounts for 56?7o of the temperature variance over a 74-yr fitting period and 45% of the variance over a 39-yr independent verification period. Relatively short-period variability (periods < 10 yr) is better reproduced in the reconstruction than is that at longer wavelengths. The reconstruction indicates that generally cool conditions prevailed across northern Fennoscandinavia around 1716-1724, 1777-1784, 1809-1818, and 1833-1842. Warm periods are reconstructed around 1766-1775 and 1788-1799 and particularly from 1826 to 1831.

Reconstructing Summer Temperatures in Northern Fennoscandinavia Back to A.D. 1700 Using Tree-Ring Data from Scots Pine

We present a methodology for the reconstruction of spatial climate patterns over the British Isles using tree rings. The predictands are mean sea-level pressures averaged over May to July for a 16-point grid centred over the British Isles. The predictors are ring widths from a network of 14 oak tree-ring chronologies in Britain and northern France. Major spatial modes of variance in the climate data are linked directly with those of the ring-width data via a series of principal component regressions.

Climate reconstruction from tree rings: Part 2, spatial reconstruction of summer mean sea‐level pressure patterns over Great Britain

Ring widths of oak trees from a network of 14 sites (13 in the British Isles and one in France) are used to reconstruct spatially averaged temperature and rainfall for England. A multiple linear regression technique is used, in which the regressors are orthogonal principal components of the ring widths. The multiple correlation coefficients are, statistically, highly significant, and the regression models' stability has been confirmed using independent data. These results demonstrate the usefulness of tree-ring widths for climate reconstruction in temperate, maritime areas.

Climate reconstruction from tree rings: Part 1, basic methodology and preliminary results for England

INTERPRETATION of proxy records of past environmental conditions derived from dated geological or biological materials is of great importance for the extension of the climatic record1,2. Precisely dated, replicated tree-ring series have been particularly useful as they provide records (usually ring-widths) dated to the individual year, for hundreds or thousands of years. Each such series, or chronology, is derived from a particular known location. Thus a network of such chronologies may be developed for a particular region and used as a proxy record of spatial and temporal climatic variations. This has been achieved in North America by Fritts et al.3,4, using principally chronologies from semi-arid areas or from near altitudinal or polar tree-lines. We report evidence here that tree-ring chronologies from sites in the British Isles will provide suitable proxy records for the reconstruction of historical temporal and spatial variation of climate.

Climatic signals in British Isles tree-ring chronologies

The results are statistically significant, and the stability of the regression models is verified using independent data. The reconstructions are more reliable for low flow events; a finding which is potentially useful for water resource planning. The reconstructions indicate a possible increase in the frequency of extreme low flows over the period 1755-1859, compared to 1860-1979, which although not of conclusive statistical significance, warrants further investigation.

J. R. Pilcher

Papers

Reconstructing Summer Temperatures in Northern Fennoscandinavia Back to A.D. 1700 Using Tree-Ring Data from Scots Pine

Climate reconstruction from tree rings: Part 2, spatial reconstruction of summer mean sea‐level pressure patterns over Great Britain

Climate reconstruction from tree rings: Part 1, basic methodology and preliminary results for England

Climatic signals in British Isles tree-ring chronologies

Riverflow reconstruction from tree rings in southern Britain