Divergence problem
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The divergence problem is an anomaly from the field of
dendroclimatology Dendroclimatology is the science of determining past climates from trees (primarily properties of the annual tree rings). Tree rings are wider when conditions favor growth, narrower when times are difficult. Other properties of the annual rings, s ...
, the study of past climate through observations of old trees, primarily the properties of their annual growth rings. It is the disagreement between instrumental temperatures (measured by
thermometer A thermometer is a device that temperature measurement, measures temperature or a temperature gradient (the degree of hotness or coldness of an object). A thermometer has two important elements: (1) a temperature sensor (e.g. the bulb of a merc ...
s) and the temperatures reconstructed from latewood densities or, in some cases, from the widths of tree rings in far northern forests. While the thermometer records indicate a substantial late 20th century warming trend, many tree rings from such sites do not display a corresponding change in their maximum latewood density. In some studies this issue has also been found with tree ring width. A temperature trend extracted from tree rings alone would not show any substantial warming since the 1950s. The temperature graphs calculated in these two ways thus "diverge" from one another, which is the origin of the term.


Discovery

The problem of changing response of some tree ring proxies to recent climate changes was first identified through research in Alaska conducted by Gordon Jacoby and
Rosanne D'Arrigo Rosanne D'Arrigo is a professor at Lamont–Doherty Earth Observatory at Columbia University known for her research into climate change using dendrochronology, or dating based on tree rings. Education and career D'Arrigo grew up in the Bronx an ...
.
Keith Briffa Keith Raphael Briffa (27 December 1952 – 29 October 2017) was a climatologist and deputy director of the Climatic Research Unit. He authored or co-authored over 130 scholarly articles, chapters and books. In his professional work, he focused on ...
's February 1998 study showed that this problem was more widespread at high northern latitudes, and warned that it had to be taken into account to avoid overestimating past temperatures.


Importance

The deviation of some tree ring proxy measurements from the instrumental record since the 1950s raises the question of the reliability of tree ring proxies in the period before the
instrumental temperature record The instrumental temperature record is a record of temperatures within Earth's climate based on direct, instrument-based measurements of air temperature and ocean temperature. Instrumental temperature records are distinguished from indirect rec ...
. The wide geographic and temporal distribution of well-preserved trees, the solid physical, chemical, and biological basis for their use, and their annual discrimination make
dendrochronology Dendrochronology (or tree-ring dating) is the scientific method of dating tree rings (also called growth rings) to the exact year they were formed. As well as dating them, this can give data for dendroclimatology, the study of climate and atmos ...
particularly important in pre-instrumental climate reconstructions. Tree ring proxies are essentially consistent with other proxy measurements for the period 1600–1950. Before around AD 1600, the uncertainty of temperature reconstructions rises due to the relative paucity of data sets and their limited geographic distribution. As of 2006, these uncertainties were considered too great to allow conclusion on whether the tree ring record diverges from other proxies during this period. In more recent studies evidence suggested that the divergence is caused by human activities, and so confined to the recent past, but use of affected proxies can lead to overestimation of past temperatures, understating the current warming trend.


Possible explanations

The explanation for the divergence problem is still unclear, but is likely to represent the impact of some other climatic variable that is important to modern northern hemisphere forests but not significant before the 1950s.
Rosanne D'Arrigo Rosanne D'Arrigo is a professor at Lamont–Doherty Earth Observatory at Columbia University known for her research into climate change using dendrochronology, or dating based on tree rings. Education and career D'Arrigo grew up in the Bronx an ...
, senior research scientist at the Tree Ring Lab at
Columbia University Columbia University (also known as Columbia, and officially as Columbia University in the City of New York) is a private research university in New York City. Established in 1754 as King's College on the grounds of Trinity Church in Manhatt ...
's
Lamont–Doherty Earth Observatory The Lamont–Doherty Earth Observatory (LDEO) is the scientific research center of the Columbia Climate School, and a unit of The Earth Institute at Columbia University. It focuses on climate and earth sciences and is located on a 189-acre (64 h ...
, hypothesizes that "beyond a certain threshold level of temperature the trees may become more stressed physiologically, especially if moisture availability does not increase at the same time." Signs suggestive of such stress are visible from space, where satellite pictures show "evidence of browning in some northern vegetation despite recent warming." Other possible explanations include that the response to recent rapid
global warming In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to E ...
might be delayed or
nonlinear In mathematics and science, a nonlinear system is a system in which the change of the output is not proportional to the change of the input. Nonlinear problems are of interest to engineers, biologists, physicists, mathematicians, and many other ...
in some fashion. The divergence might represent changes to other climatic variables to which tree rings are sensitive, such as delayed snowmelt and changes in seasonality. Growth rates could depend more on annual maximum or minimum temperatures, especially in temperature limited growth regions (i.e. high latitudes and altitudes). Another possible explanation is
global dimming Global dimming is the reduction in the amount of global direct irradiance at the Earth's surface that has been observed since systematic measurements began in the 1950s. The effect varies by location, but worldwide it has been estimated to be of ...
due to atmospheric
aerosol An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be natural or Human impact on the environment, anthropogenic. Examples of natural aerosols are fog o ...
s. In 2012, Brienen et al. proposed that the divergence problem was largely an artifact of sampling large living trees.


See also

*
Climatic Research Unit email controversy The Climatic Research Unit email controversy (also known as "Climategate") began in November 2009 with the hacking of a server at the Climatic Research Unit (CRU) at the University of East Anglia (UEA) by an external attacker, copying thousa ...
* Climatic Research Unit documents#Climate reconstruction graph


References


External links


Curiosity Rises With Trees' Strange Growth Spurt
NPR National Public Radio (NPR, stylized in all lowercase) is an American privately and state funded nonprofit media organization headquartered in Washington, D.C., with its NPR West headquarters in Culver City, California. It differs from other ...
, March 28, 2010 {{DEFAULTSORT:Divergence Problem Paleoclimatology Dendrology