In the assessment of climate variability and change, the term “weather noise” often comes up. The idea is that a long term climate change is embedded within much more variable shorter term weather patterns, and that long term trend assessments are needed in order to filter out these shorter term effects.
However, this concept of “weather noise’ has several problems with it:
1. There is an implication with the use of this term that there well defined separations in time scales between weather and climate. That this is false has been shown in numerous studies summarized, for example, in
Rial, J., R.A. Pielke Sr., M. Beniston, M. Claussen, J. Canadell, P. Cox, H. Held, N. de Noblet-Ducoudre, R. Prinn, J. Reynolds, and J.D. Salas, 2004: Nonlinearities, feedbacks and critical thresholds within the Earth’s climate system. Climatic Change, 65, 11-38
where we wrote
“The Earth’s climate system is highly nonlinear: inputs and outputs are not proportional, change is often episodic and abrupt, rather than slow and gradual, and multiple equilibria are the norm.”
The paper
Seidel D. J., J. R. Lanzante (2004), An assessment of three alternatives to linear trends for characterizing global atmospheric temperature changes, J. Geophys. Res., 109, D14108, doi:10.1029/2003JD004414,
illustrates one way to deal with this nonlinearity in the climate system where they write
“Historical changes in global atmospheric temperature are typically estimated using simple linear trends. This paper considers three alternative simple statistical models, each involving breakpoints (abrupt changes): a flat steps model, in which all changes occur abruptly; a piecewise linear model; and a sloped steps model, incorporating both abrupt changes and slopes during the periods between breakpoints….These models are applied to the instrumental record of global monthly temperature anomalies at the surface and to the radiosonde and satellite records for the troposphere and stratosphere. The alternative models often provide a better fit to the observations than the simple linear model…..Results for tropospheric data suggest that it is reasonable to consider most of the warming during 1958–2001 to have occurred at the time of the abrupt climate regime shift in 1977.”
What is really meant by the term “weather noise” is provide by the definition of the term “climate change” by the IPCC glossary. “Climate change” by their definition is
Climate change: Climate change refers to a change in the state of the climate that can be identified (e.g., by using statistical tests) by changes in the mean and/or the variability of its properties, and that persists for an extended period, typically decades or longer.
Thus “weather noise” are those changes in the mean and/or the variability of its properties, and that occur on time scales less than decades. It should more accurately be called “short term climate variability”.
2. More importantly, with respect to the focus of this weblog, there is a distinction between the short term variabiltiy of climate variables, such as temperature at 2m, precipitation, etc, which do not include mass weighted quantities, and global climate heat content, global average sea level, etc which include mass as part of their definition (see also). These mass weighted quantities are integral climate variables since they are volume averages. Since they are integral quantities, they are a type of low pass filter in that high frequency variations are naturally filtered out of the signal.
The use of integral climate variables is particularly useful with respect to monitoring global warming and cooling (i.e. “global warming”). The use of these variables permits differences in time slices (i.e. “snapshots”) of the values to indicate what has been the climate system change in heat content for that time period. There is no need to compute a linear trend. Indeed, if the heat suddenly was reduced to an earlier value, the “trend” up to that time is irrelevant.
As I wrote in
Pielke Sr., R.A., 2003: Heat storage within the Earth system. Bull. Amer. Meteor. Soc., 84, 331-335
“A snapshot at any time documents the accumulated heat content and its change since the last assessment. Unlike temperature, at some specific level of the ocean, land, or the atmosphere, in which there is a time lag in its response to radiative forcing, there are no time lags associated with heat changes.”
Thus, while sampling issues are certainly relevant, the concept of “weather noise” itself is not particularly relevant to the use of the global average upper ocean heat content (or the global average sea level rise) to assess climate change.
