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The latest response from the climate community comes in State of the Climate in 2008, a special supplement to the current (August) issue of the Bulletin of the American Meteorological Society. Climate researcher Jeff Knight and eight colleagues at the Met Office Hadley Centre in Exeter, U.K., first establish that —at least in one leading temperature record— greenhouse warming has been stopped in its tracks for the past 10 years.
In the HadCRUT3 temperature record, the world warmed by 0.07°C±0.07°C from 1999 through 2008, not the 0.20°C expected by the Intergovernmental Panel on Climate Change. Corrected for the natural temperature effects of El Niño and its sister climate event La Niña, the decade’s trend is a perfectly flat 0.00°C.
A Hiatus of the Greenhouse Effect
The rate at which the global average surface air temperature (Ts) increases has slowed down during the past few decades. This so-called hiatus, pause, or slowdown of global warming has inspired investigations into its potential causes worldwide.
Although some researchers doubted the existence of a global warming hiatus because of coverage bias, artificial inconsistency, and a change point analysis of instrumental Ts records, it is now accepted that a recent warming deceleration can be clearly observed. There are two primary hypotheses to explain the recent slowdown of the upward trend in Ts.
Both hypotheses attempt to explain the contradiction between the trendless Ts variation and the intensifying anthropogenic greenhouse effect resulting from the steadily increasing emission of greenhouse gases (GHGs). The first attributes the warming hiatus to external radiative forcings, such as decreasing solar irradiance, increasing tropospheric and stratospheric aerosols, reduced stratospheric water vapor, and several small volcanic eruptions. The warming effect of increasing GHGs is largely cancelled out by the decreasing solar shortwave radiation received by the Earth’s surface.
The second considers the warming pause to be a result of internal oceanic and/or atmospheric decadal variabilities against the centennial warming trend, in which two leading theories are proposed. One asserts that the recent warming hiatus likely results from a La Niña-like state or a negative phase of Interdecadal Pacific Oscillation (IPO) associated with the cooling tropical Pacific sea surface temperature (SST) and the increasing Pacific trade winds. This theory is supported by the successful simulation of the warming hiatus by nudging the tropical pacific SST or trade winds relative to observations.
The other suggests that the warming hiatus is accompanied by increasing heat uptake in global deep oceans. This extra heat, which originates from a positive radiative imbalance at the top of the atmosphere (TOA), is reserved in the deep oceans instead of warming the Earth’s skin. Note that both aforementioned hypotheses indeed include an enhancing greenhouse effect in which more heat is captured by the Earth–atmosphere system.
The main difference between them is how this additional energy is prevented from warming the Earth’s surface.
Volcanic Contributions to Decadal Changes in Tropospheric Temperature
Despite continued growth in atmospheric levels of greenhouse gases, global mean surface and tropospheric temperatures show slower warming since 1998. Possible explanations for this “warming hiatus” include internal climate variability, external cooling influences, and observational errors.
One contributory factor to the relatively muted surface warming – early 21st century volcanic forcing – has been examined in several modelling studies. Here we present the first analysis of the impact of recent volcanic forcing on tropospheric temperature, and the first observational assessment of the significance of early 21st century volcanic signals.
the Pacific and the ongoing warming hiatus
A key component of the global hiatus that has been identified is cool eastern Pacific sea surface temperature, but it is unclear how the ocean has remained relatively cool there in spite of ongoing increases in radiative forcing.
Here we show that a pronounced strengthening in Pacific trade winds over the past two decades—unprecedented in observations/reanalysis data and not captured by climate models—is sufficient to account for the cooling of the tropical Pacific and a substantial slowdown in surface warming through increased subsurface ocean heat uptake.
Making sense of the early 2000s warming slowdown
Research into the nature and causes of the slowdown has triggered improved understanding of observational biases, radiative forcing and internal variability. This has led to widespread recognition that modulation by internal variability is large enough to produce a significantly reduced rate of surface temperature increase for a decade or even more — particularly if internal variability is augmented by the externally driven cooling caused by a succession of volcanic eruptions.
The legacy of this new understanding will certainly outlive the recent warming slowdown. This is particularly true in the embryonic field of decadal climate prediction, where the challenge is to simulate how the combined effects of external forcing and internal variability produce the time-evolving regional climate we will experience over the next ten years
Artificial Amplification of Warming Trends Across the Mountains of the Western United States
Here we critically evaluate this network’s temperature observations and show that extreme warming observed at higher elevations is the result of systematic artifacts and not climatic conditions. With artifacts removed, the network’s 1991–2012 minimum temperature trend decreases from +1.16°C/decade to +0.106°C/decade and is statistically indistinguishable from lower elevation trends.
Moreover, longer-term widely used gridded climate products propagate the spurious temperature trend, thereby amplifying 1981–2012 western U.S. elevation-dependent warming by +217 to +562%. In the context of a warming climate, this artificial amplification of mountain climate trends has likely compromised our ability to accurately attribute climate change impacts across the mountainous western U.S.
An overview of the Global Historical Climatology Network
Since the early 1990s the Global Historical Climatology Network-Monthly (GHCN-M) data set has been an internationally recognized source of data for the study of observed variability and change in land surface temperature. It provides monthly mean temperature data for 7280 stations from 226 countries and territories, ongoing monthly updates of more than 2000 stations to support monitoring of current and evolving climate conditions, and homogeneity adjustments to remove non-climatic influences that can bias the observed temperature record.
Homogenization of Temperature Series via Pairwise Comparisons
An automated homogenization algorithm based on the pairwise comparison of monthly temperature series is described. The algorithm works by forming pairwise difference series between serial monthly temperature values from a network of observing stations. Each difference series is then evaluated for undocumented shifts, and the station series responsible for such breaks is identified automatically.
The algorithm also makes use of station history information, when available, to improve the identification of artificial shifts in temperature data. In addition, an evaluation is carried out to distinguish trend inhomogeneities from abrupt shifts. When the magnitude of an apparent shift attributed to a particular station can be reliably estimated, an adjustment is made for the target series.
NASA-GLOBAL SURFACE TEMPERATURE CHANGE
Global satellite measurements of night lights allow the possibility for an additional check on the magnitude of the urban influence on global temperature analyses. We describe in this section a procedure in which all stations located in areas with night light brightness exceeding a value (see reference) that approximately divides the stations into two categories: rural and urban or periurban.
The standard GISS global temperature analysis now adjusts the long‐term trends of stations located in regions with night light brightness exceeding this limit to agree with the long‐term trend of nearby rural stations.
Urban warming in Japanese cities and its relation to climate change monitoring
Urban warming is quite conspicuous in large cities in Japan, reflecting their rapid growth during the last century. The temperature increase in Tokyo, where the meteorological observatory is located in the central business area of the city, has been about 3 °C/century. The warming rate is larger for nighttime (minimum) temperatures than for daytime (maximum) temperatures, in agreement with the general features of the urban heat island.
In particular, the annual extreme minimum temperature has increased at a rate exceeding 10 °C/century in some cities in Hokkaido, corresponding to drastic changes from a small city or wild land to large or medium-sized cities at these sites.
From the viewpoint of climate change monitoring, urban warming can be a biasing factor that may contaminate data used for monitoring the background temperature change. An analysis using 29 years of AMeDAS data revealed the existence of anomalous temperature changes, not only at densely inhabited sites but also at stations with relatively small populations in the surrounding area.
Urban heat island intensity observed at Beijing and Wuhan stations
Annual and seasonal urbanization-induced warming for the two periods at Beijing and Wuhan stations is also generally significant, with the annual urban warming accounting for about 65∼80% of the overall warming in 1961∼2000 and about 40∼61% of the overall warming in 1981∼2000.
Increase in annual mean SAT during 1961∼2000 reaches 0.32°C/10 yr. and 0.31°C/10 yr. respectively for Beijing Station and Wuhan Station, but it is only 0.06°C/10 yr. and 0.11°C/10 yr. for averages of the rural stations around the two cities, indicating that the temperature increase at the two city stations is mostly caused by urban warming.
A case study of Mumbai
The study reveals that the rapid and potent day time heating of the air over the costal region contribute to the rise of day temperatures at Colaba, while the adjacent water stretch remains less warm. This natural contradiction contributes to the rise of UHI. It further reveals that city and suburbs of Mumbai remain warmer than the peripheral areas namely Alibag and Dahanu.
This difference is due to not only natural factors but also anthropogenic interference. The progressive replacement of natural surface with built up surface and increasing population pressure through the process of urbanisation modified the physical and chemical properties of city and suburbs of Mumbai. This negatively impacted the environment and cumulative effect of all this is the creation of UHI.
The plot files contain 9 individual graphs, arranged in a 3x3 matrix. The first column of graphs, contain 2-D colored symbol graphs of the actual monthly data for the entire period of record for A) the (Q)uality (C)ontrolled (U)nadjusted (QCU) data, B) the (Q)uality (C)ontrolled (A)djusted (QCA) data, and C) the differences between QCA and QCU monthly data.
The second column of graphs contain histograms of the monthly data for QCU, QCA, and (QCA-QCU) respectively.
The third column of graphs depict annual anomalies and their associated trend line for QCU and QCA, and the differences in the annual anomalies for QCAand QCU. Detailed axis titles and units are displayed in the title of each graph
More urban heat; less summer fog, on California coast
The summer fog that shrouds coastal southern California – what locals call the June Gloom – is being driven up into the sky by urban sprawl, according to scientists who have studied 67 years of cloud heights and urban growth in the region. Less fog may, at first, seem like a good thing. But less fog is bad news for native plants in the coastal hills and mountains, which depend on the cool fog as their only source of water during the rainless summer months. So less fog means warmer, drier, less healthy hillsides and potentially more fires.(...)
Another surprising discovery was what Williams and his colleagues saw on the undeveloped airfields on islands off the coast of southern California: Cloud height lowered over the last 60 years.
The temperature inversion layer in the atmosphere that holds the moist marine air close to the ground (the same inversion that can hold smog near the ground) actually has been getting stronger, making it harder to break up the clouds, Williams said. It’s the opposite trend to what was seen in the urban areas, which suggests another long-term climate trend, like global warming, is causing the change, he added. “In the absence of urbanization we might see an increase in fog,” he concluded.
Urbanization causes increased cloud base height and decreased fog in coastal Southern California