Mads Faurschou Knudsen

Tracking the Atlantic Multidecadal Oscillation through the last 8,000 years.

Understanding the internal ocean variability and its influence on climate is imperative for society. A key aspect concerns the enigmatic Atlantic Multidecadal Oscillation (AMO), a feature defined by a 60- to 90-year variability in North Atlantic sea-surface temperatures. The nature and origin of the AMO is uncertain, and it remains unknown whether it represents a persistent periodic driver in the climate system, or merely a transient feature. Here, we show that distinct, ∼55- to 70-year oscillations characterized the North Atlantic ocean-atmosphere variability over the past 8,000 years. We test and reject the hypothesis that this climate oscillation was directly forced by periodic changes in solar activity. We therefore conjecture that a quasi-persistent ∼55- to 70-year AMO, linked to internal ocean-atmosphere variability, existed during large parts of the Holocene. Our analyses further suggest that the coupling from the AMO to regional climate conditions was modulated by orbitally induced shifts in large-scale ocean-atmosphere circulation.

Lifespan of mountain ranges scaled by feedbacks between landsliding and erosion by rivers

An important challenge in geomorphology is the reconciliation of the high fluvial incision rates observed in tectonically active mountain ranges with the long-term preservation of significant mountain-range relief in ancient, tectonically inactive orogenic belts. River bedrock erosion and sediment transport are widely recognized to be the principal controls on the lifespan of mountain ranges. But the factors controlling the rate of erosion and the reasons why they seem to vary significantly as a function of tectonic activity remain controversial. Here we use computational simulations to show that the key to understanding variations in the rate of erosion between tectonically active and inactive mountain ranges may relate to a bidirectional coupling between bedrock river incision and landslides. Whereas fluvial incision steepens surrounding hillslopes and increases landslide frequency, landsliding affects fluvial erosion rates in two fundamentally distinct ways. On the one hand, large landslides overwhelm the river transport capacity and cause upstream build up of sediment that protects the river bed from further erosion. On the other hand, in delivering abrasive agents to the streams, landslides help accelerate fluvial erosion. Our models illustrate how this coupling has fundamentally different implications for rates of fluvial incision in active and inactive mountain ranges. The coupling therefore provides a plausible physical explanation for the preservation of significant mountain-range relief in old orogenic belts, up to several hundred million years after tectonic activity has effectively ceased.

Evidence for external forcing of the Atlantic Multidecadal Oscillation since termination of the Little Ice Age

The Atlantic Multidecadal Oscillation (AMO) represents a significant driver of Northern Hemisphere climate, but the forcing mechanisms pacing the AMO remain poorly understood. Here we use the available proxy records to investigate the influence of solar and volcanic forcing on the AMO over the last ~450 years. The evidence suggests that external forcing played a dominant role in pacing the AMO after termination of the Little Ice Age (LIA; ca. 1400–1800), with an instantaneous impact on mid-latitude sea-surface temperatures that spread across the North Atlantic over the ensuing ~5 years. In contrast, the role of external forcing was more ambiguous during the LIA. Our study further suggests that the Atlantic Meridional Overturning Circulation is important for linking external forcing with North Atlantic sea-surface temperatures, a conjecture that reconciles two opposing theories concerning the origin of the AMO.

Taking the pulse of the Sun during the Holocene by joint analysis of 14C and 10Be

We have studied solar variations during the Holocene (i.e., last similar to 11,700 yr) by combining a new model of the Earths dipole moment with C-14 data from the IntCal04 record and 10 Be data from the GRIP ice core. Joint spectral analysis of the two nuclide records suggests that the periodic behavior of the Sun was particularly pronounced between 6000-4500 yr BP and 3000-2000 yr BP, with dominating periodicities of similar to 88, similar to 150, similar to 220, and similar to 400 years, while this rhythmic behavior faded during other time intervals. The fact that the two reconstructions, based on radionuclides with distinct geochemical properties, agree with respect to both the frequency and timing of the periodic behavior, strongly suggests that they reflect the actual behavior of the Sun. Subtle but systematic differences between the amplitude spectra may point to an interplay between the climate system and the similar to 220- and similar to 400-year solar cycles during intervals when these were particularly prominent. Citation: Knudsen, M. F., P. Riisager, B. H. Jacobsen, R. Muscheler, I. Snowball, and M.-S. Seidenkrantz (2009), Taking the pulse of the Sun during the Holocene by joint analysis of 14 C and 10 Be, Geophys. Res. Lett., 36, L16701, doi: 10.1029/2009GL039439. (Less)

Solar forcing of Holocene summer sea-surface temperatures in the northern North Atlantic

Mounting evidence from proxy records suggests that variations in solar activity have played a significant role in triggering past climate changes. However, the mechanisms for sun-climate links remain a topic of debate. Here we present a high-resolution summer sea-surface temperature (SST) record covering the past 9300 yr from a site located at the present-day boundary between polar and Atlantic surface-water masses. The record is age constrained via the identification of 15 independently dated tephra markers from terrestrial archives, circumventing marine reservoir age variability problems. Our results indicate a close link between solar activity and SSTs in the northern North Atlantic during the past 4000 yr; they suggest that the climate system in this area is more susceptible to the influence of solar variations during cool periods with less vigorous ocean circulation. Furthermore, the high-resolution SST record indicates that climate in the North Atlantic regions follows solar activity variations on multidecadal to centennial time scales.

Is there a link between Earth's magnetic field and low-latitude precipitation?

Some studies indicate that the solar modulation of galactic cosmic ray (GCR) particles has profound consequences for Earth9s climate system. A corollary of the GCR-climate theory involves a link between Earth9s magnetic field and climate, since the geomagnetic field also modulates the GCR flux reaching Earth9s atmosphere. In this study, we explore this potential geomagnetic-climate link by comparing a new reconstruction of the Holocene geomagnetic dipole moment with high-resolution speleothem data from China and Oman. The speleothem δ 18 O data represent proxy records for past precipitation in low-latitude regions, which is a climate parameter that is likely to have been sensitive to variations in the GCR flux modulated by the dipole moment. Intriguingly, we observe a relatively good correlation between the high-resolution speleothem δ 18 O records and the dipole moment, suggesting that Earth9s magnetic field to some degree influenced low-latitude precipitation in the past. In addition to supporting the notion that variations in the geomagnetic field may have influenced Earth9s climate in the past, our study also provides some degree of support for the controversial link between GCR particles, cloud formation, and climate.

Paleomagnetic evidence from Cape Verde Islands basalts for fully reversed excursions in the Brunhes Chron

Abstract In this study we present paleomagnetic data from two lava sequences on Santo Antao, Cape Verde Islands: the Tarrafal and Agua Nova profiles from which 63 and 43 lava flows were sampled, respectively. Previous 40Ar/39Ar ages have constrained the two profiles to the Brunhes Chron, which is in accordance with the normal polarity of the majority of the flows. Some individual lava flows as well as flow sequences with virtual geomagnetic poles deviating more than 45° from the geographic North Pole are interpreted to represent geomagnetic excursions. Based on interpretation of the directional data three excursions are recorded in the Tarrafal profile (T-I, T-II, and T-III) and four in the Agua Nova profile (AG-I, AG-II, AG-III, and AG-IV). Both 40Ar/39Ar results and paleomagnetic directional data indicate that an excursion defined by eight flows in the Tarrafal profile (T-I) is the same as one recorded in six flows in the Agua Nova profile (AG-I). This excursion, which passes the reversal test with classification C, is the first geomagnetic excursion in the Brunhes Chron to pass the reversal test. The 40Ar/39Ar ages suggest that this excursion could represent chryptochron C1n-1 (0.493–0.504 Ma), the only chryptochron or ‘tiny wiggle’ identified in the marine magnetic anomaly record for the Brunhes Chron. The remaining excursions may not yet be unambiguously correlated between the two profiles. Another excursion recorded by two flows in the top of the Tarrafal profile (T-III) also displays completely reversed polarity.

Observational evidence for enhanced magnetic activity of superflare stars

Superflares are large explosive events on stellar surfaces one to six orders-of-magnitude larger than the largest flares observed on the Sun throughout the space age. Due to the huge amount of energy released in these superflares, it has been speculated if the underlying mechanism is the same as for solar flares, which are caused by magnetic reconnection in the solar corona. Here, we analyse observations made with the LAMOST telescope of 5,648 solar-like stars, including 48 superflare stars. These observations show that superflare stars are generally characterized by larger chromospheric emissions than other stars, including the Sun. However, superflare stars with activity levels lower than, or comparable to, the Sun do exist, suggesting that solar flares and superflares most likely share the same origin. The very large ensemble of solar-like stars included in this study enables detailed and robust estimates of the relation between chromospheric activity and the occurrence of superflares.

Grand solar minima and maxima deduced from 10 Be and 14 C: magnetic dynamo configuration and polarity reversal

Aims. This study aims to improve our understanding of the occurrence and origin of grand solar maxima and minima. Methods. We first investigate the statistics of peaks and dips simultaneously occurring in the solar modulation potentials reconstructed using the Greenland Ice Core Project (GRIP) 10 Be and IntCal13 14 C records for the overlapping time period spanning between 1650 AD to 6600 BC. Based on the distribution of these events, we propose a method to identify grand minima and maxima periods. By using waiting time distribution analysis, we investigate the nature of grand minima and maxima periods identified based on the criteria as well as the variance and significance of the Hale cycle during these kinds of events throughout the Holocene epoch. Results. Analysis of grand minima and maxima events occurring simultaneously in the solar modulation potentials, reconstructed based on the 14 C and the 10 Be records, shows that the majority of events characterized by periods of moderate activity levels tend to last less than 50 years: grand maxima periods do not last longer than 100 years, while grand minima can persist slightly longer. The power and the variance of the 22-year Hale cycle increases during grand maxima and decreases during grand minima, compared to periods characterized by moderate activity levels. Conclusions. We present the first reconstruction of the occurrence of grand solar maxima and minima during the Holocene based on simultaneous changes in records of past solar variability derived from tree-ring 14 C and ice-core 10 Be, respectively. This robust determination of the occurrence of grand solar minima and maxima periods will enable systematic investigations of the influence of grand solar minima and maxima episodes on Earth’s climate.

Palaeomagnetic distortion modelling and possible recovery by inversion

Abstract A robust finite-element technique is presented for computation of both the internal demagnetization effects and magnetic terrain effects in bodies with arbitrary shape and arbitrary susceptibility distribution. This method facilitates a flexible analysis of the palaeomagnetic deflection problem. Tests on geologically realistic settings of highly magnetic rocks demonstrate that deflections of several degrees may occur even for relatively simple two-dimensional models. Similarly, the magnetic intensity may well be biased by 5–15% by demagnetization effects. The present paper focuses on deflections and intensity variations inside the magnetized body, where we find a systematic shallowing of inclination for bodies with a horizontal elongation. Because the bodies sampled at a typical palaeomagnetic site will have a dominant direction of elongation, the magnetic deflection effect will tend to impose a systematic bias which doesn’t average out. An inversion-based procedure for elimination of the deflection effect is presented. It requires that the magnetic body is quite homogeneous and that its surface geometry is known, as may be the case for historical lava flows. Tests demonstrate that in order to recover both ambient palaeofield direction and the effective susceptibility at blocking temperature it is necessary to sample near strong topographic elements in the magnetic body. Since the surface geometry rarely is known it is proposed as an alternative to inversion that an effective susceptibility is assessed and a horizontal slab correction is applied for samples taken far from topographical features. When shape geometry is unknown and no correction applied, palaeomagnetic conclusions must take into account the possible bias from internal demagnetization and magnetic terrain effects.