{"id":430588,"date":"2026-03-12T09:58:00","date_gmt":"2026-03-12T08:58:00","guid":{"rendered":"https:\/\/climatescience.press\/?p=430588"},"modified":"2026-03-12T09:58:02","modified_gmt":"2026-03-12T08:58:02","slug":"the-modern-co%e2%82%82-spike-looks-scarier-than-it-really-is","status":"publish","type":"post","link":"https:\/\/climatescience.press\/?p=430588","title":{"rendered":"The Modern CO\u2082 Spike Looks Scarier Than It Really Is"},"content":{"rendered":"<div class=\"wp-block-image\">\n<figure class=\"aligncenter size-large\"><img data-recalc-dims=\"1\" loading=\"lazy\" decoding=\"async\" width=\"687\" height=\"1024\" data-attachment-id=\"430590\" data-permalink=\"https:\/\/climatescience.press\/?attachment_id=430590\" data-orig-file=\"https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/0-The-Modern-CO%E2%82%82-Spike-Looks-Scarier-Than-It-Really-Is.jpg?fit=784%2C1168&amp;ssl=1\" data-orig-size=\"784,1168\" data-comments-opened=\"1\" data-image-meta=\"{&quot;aperture&quot;:&quot;0&quot;,&quot;credit&quot;:&quot;&quot;,&quot;camera&quot;:&quot;&quot;,&quot;caption&quot;:&quot;&quot;,&quot;created_timestamp&quot;:&quot;0&quot;,&quot;copyright&quot;:&quot;&quot;,&quot;focal_length&quot;:&quot;0&quot;,&quot;iso&quot;:&quot;0&quot;,&quot;shutter_speed&quot;:&quot;0&quot;,&quot;title&quot;:&quot;&quot;,&quot;orientation&quot;:&quot;0&quot;}\" data-image-title=\"0 The Modern CO\u2082 Spike Looks Scarier Than It Really Is\" data-image-description=\"\" data-image-caption=\"\" data-large-file=\"https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/0-The-Modern-CO%E2%82%82-Spike-Looks-Scarier-Than-It-Really-Is.jpg?fit=687%2C1024&amp;ssl=1\" src=\"https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/0-The-Modern-CO%E2%82%82-Spike-Looks-Scarier-Than-It-Really-Is.jpg?resize=687%2C1024&#038;ssl=1\" alt=\"\" class=\"wp-image-430590\" srcset=\"https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/0-The-Modern-CO%E2%82%82-Spike-Looks-Scarier-Than-It-Really-Is.jpg?resize=687%2C1024&amp;ssl=1 687w, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/0-The-Modern-CO%E2%82%82-Spike-Looks-Scarier-Than-It-Really-Is.jpg?resize=201%2C300&amp;ssl=1 201w, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/0-The-Modern-CO%E2%82%82-Spike-Looks-Scarier-Than-It-Really-Is.jpg?resize=768%2C1144&amp;ssl=1 768w, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/0-The-Modern-CO%E2%82%82-Spike-Looks-Scarier-Than-It-Really-Is.jpg?resize=640%2C953&amp;ssl=1 640w, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/0-The-Modern-CO%E2%82%82-Spike-Looks-Scarier-Than-It-Really-Is.jpg?w=784&amp;ssl=1 784w\" sizes=\"auto, (max-width: 687px) 100vw, 687px\" \/><\/figure>\n<\/div>\n\n\n<p class=\"wp-block-paragraph\">Directly splicing the modern Mauna Loa record (~427 ppm in 2025) onto Antarctic ice-core data creates a visually alarming \u201chockey-stick\u201d spike. <\/p>\n\n\n\n<p class=\"wp-block-paragraph\">But this comparison is apples- to- oranges because ice- core proxies (especially from low-accumulation sites like Dome C or Vostok) heavily smooth atmospheric signals over 100\u2013300+ years due to firn diffusion. Rapid modern changes get \u201cmuted.\u201d<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Ice cores from Antarctica provide the primary proxy records for past atmospheric CO\u2082 concentrations, extending back up to ~800,000 years. <\/p>\n\n\n\n<p class=\"wp-block-paragraph\">However, different drill sites vary significantly in temporal resolution (how finely they capture short-term changes) due to snow accumulation rates, firn diffusion (gas mixing in the porous snow\/ice transition layer before bubbles close off), and site-specific conditions like temperature and wind. <\/p>\n\n\n\n<p class=\"wp-block-paragraph\">This leads to varying degrees of smoothing (averaging or damping of rapid fluctuations), with low-accumulation interior sites acting as low-pass filters over centuries, while high-accumulation coastal sites preserve decadal-scale details but shorter records.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Here\u2019s a comparison of the main sites discussed in paleoclimate literature (e.g., EPICA Dome C, Vostok, Law Dome, and others like WAIS Divide or Siple Dome):<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>EPICA Dome C (EDC) \u2014 East Antarctic Plateau (interior, very low accumulation ~2.5 cm water eq.\/yr, cold ~ -54\u00b0C)\n<ul class=\"wp-block-list\">\n<li>Longest continuous record: ~800,000 years (full glacial-interglacial cycles).<\/li>\n\n\n\n<li>Temporal resolution\/smoothing: High smoothing, gas age distribution ~200\u2013550 years (wider during glacials due to thicker firn); often ~100\u2013300+ years effective averaging.<\/li>\n\n\n\n<li>Strengths: Excellent for long-term trends and glacial-interglacial CO\u2082 ranges (e.g., 180\u2013300 ppm).<\/li>\n\n\n\n<li>Limitations: Rapid decadal\/centennial changes (like modern rise) heavily muted; used in composites for deep time.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li>Vostok \u2014 East Antarctic Plateau (interior, low accumulation ~2\u20133 cm water eq.\/yr)\n<ul class=\"wp-block-list\">\n<li>Record: ~420,000 years (classic early deep core).<\/li>\n\n\n\n<li>Temporal resolution\/smoothing: Similar to Dome C, ~several centuries (high firn diffusion due to low accumulation); often the most muted among major records.<\/li>\n\n\n\n<li>Strengths: Pioneering record showing CO\u2082-temperature correlations.<\/li>\n\n\n\n<li>Limitations: Poorer resolution than Dome C; larger sample spacing in older sections.<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li>Law Dome \u2014 Coastal East Antarctica (high accumulation ~60\u2013110 cm water eq.\/yr, milder ~ -20\u00b0C)\n<ul class=\"wp-block-list\">\n<li>Record: High-resolution for recent past (~last 2,000\u201310,000 years; best overlap with instrumental era).<\/li>\n\n\n\n<li>Temporal resolution\/smoothing: Minimal, gas age distribution ~8\u201314 years (FWHM); captures multidecadal variability (e.g., ~10 ppm swings in Holocene).<\/li>\n\n\n\n<li>Strengths: Best for bridging ice cores to modern Mauna Loa data; shows finer details like ~10 ppm drop ~1600 AD.<\/li>\n\n\n\n<li>Limitations: Shorter depth; not for deep paleo (beyond ~few kyr reliably for gases).<\/li>\n<\/ul>\n<\/li>\n\n\n\n<li>Other notable sites (for context):\n<ul class=\"wp-block-list\">\n<li>WAIS Divide (West Antarctica, moderate-high accumulation ~20 cm water eq.\/yr): ~19-year smoothing; good millennial\/centennial resolution over last ~1,000\u20132,000 years; sometimes 0\u20136 ppm offsets from Law Dome.<\/li>\n\n\n\n<li>Siple Dome (West Antarctica): Higher resolution than interior sites; useful for deglaciation\/Holocene but occasional in situ CO\u2082 artifacts.<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">_____________________________________________________________________________________<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><\/p>\n\n\n\n<p class=\"has-large-font-size wp-block-paragraph\"><strong>Proxy CO\u2082 Chronicles: Muted Modern CO\u2082<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">From <a href=\"https:\/\/georox.substack.com\/p\/proxy-co-chronicles-muted-modern\">GeoRox Substack<\/a><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">By <a href=\"https:\/\/substack.com\/@georox\">Renee Hannon<\/a><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">We often see graphs of past CO\u2082 concentrations from ice cores simply sliced onto modern instrumental measurements. The modern spike looks dramatic, even alarming. But is that a fair comparison?<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">In this series, we\u2019ve discussed how paleo-CO\u2082 proxies contain uncertainties and, more importantly, temporal smoothing. So before comparing modern CO\u2082 to the past, we need to view it through the lens of proxy smoothing.<a target=\"_blank\" href=\"https:\/\/substackcdn.com\/image\/fetch\/$s_!ldkd!,f_auto,q_auto:good,fl_progressive:steep\/https%3A%2F%2Fsubstack-post-media.s3.amazonaws.com%2Fpublic%2Fimages%2Fcdd844db-6bbe-4c2a-a5e8-2837e3bfa065_1194x676.jpeg\" rel=\"noreferrer noopener\"><\/a><\/p>\n\n\n\n<figure class=\"wp-block-image size-large\"><img data-recalc-dims=\"1\" loading=\"lazy\" decoding=\"async\" width=\"723\" height=\"410\" data-attachment-id=\"430594\" data-permalink=\"https:\/\/climatescience.press\/?attachment_id=430594\" data-orig-file=\"https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-65.png?fit=1194%2C676&amp;ssl=1\" data-orig-size=\"1194,676\" data-comments-opened=\"1\" data-image-meta=\"{&quot;aperture&quot;:&quot;0&quot;,&quot;credit&quot;:&quot;&quot;,&quot;camera&quot;:&quot;&quot;,&quot;caption&quot;:&quot;&quot;,&quot;created_timestamp&quot;:&quot;0&quot;,&quot;copyright&quot;:&quot;&quot;,&quot;focal_length&quot;:&quot;0&quot;,&quot;iso&quot;:&quot;0&quot;,&quot;shutter_speed&quot;:&quot;0&quot;,&quot;title&quot;:&quot;&quot;,&quot;orientation&quot;:&quot;0&quot;}\" data-image-title=\"image\" data-image-description=\"\" data-image-caption=\"\" data-large-file=\"https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-65.png?fit=723%2C410&amp;ssl=1\" src=\"https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-65.png?resize=723%2C410&#038;ssl=1\" alt=\"\" class=\"wp-image-430594\" srcset=\"https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-65.png?resize=1024%2C580&amp;ssl=1 1024w, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-65.png?resize=300%2C170&amp;ssl=1 300w, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-65.png?resize=768%2C435&amp;ssl=1 768w, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-65.png?resize=640%2C362&amp;ssl=1 640w, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-65.png?w=1194&amp;ssl=1 1194w\" sizes=\"auto, (max-width: 723px) 100vw, 723px\" \/><figcaption class=\"wp-element-caption\"><em>Figure 1: Mauna Loa CO\u2082 concentrations spliced onto CO\u2082 data from Antarctic ice cores (Bereiter, 2015).<\/em><\/figcaption><\/figure>\n\n\n\n<h3 class=\"wp-block-heading\">Paleo-CO\u2082 Uncertainty<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Antarctic ice cores remain the gold standard for reconstructing atmospheric CO\u2082 concentrations over the past 800,000 years.&nbsp;<a href=\"https:\/\/substack.com\/@georox\/p-187436988\">Boron isotopes<\/a>&nbsp;(\u03b4\u00b9\u00b9B) in planktonic foraminifera also provide robust estimates of past CO\u2082 through their relationship with seawater pH. Both methods, however, have limitations.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">For ice cores, analytical precision is excellent \u2014 typically around \u00b11 ppm. Inter-core offsets of 3\u201310 ppm occur (for example, WAIS Divide vs. Law Dome), likely due to subtle in-situ effects or extraction differences. In the context of a 170\u2013300 ppm glacial-interglacial range, that represents at most 3% relative uncertainty. But analytical precision is not the main issue.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The dominant limitation is temporal resolution. Temporal resolution is a key issue for older ice cores and boron isotopes due to averaging or smoothing of atmospheric equivalent CO\u2082 over&nbsp;<strong>100 to 300+<\/strong>&nbsp;years.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Firn Smoothing is a Big Deal<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Before bubbles are sealed in ice, air diffuses through the porous firn layer. This process blends atmospheric signals over time. At high accumulation sites like Law Dome, smoothing may be limited to a few decades. However, high resolution ice cores only exist over thousands of years.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">At low accumulation Antarctic sites, which provide the deepest and longest records, gas age distributions typically smooth atmospheric CO\u2082 over ~100 to 300+ years (Kohler, 2011; Nehrbass-Ahles, 2020).<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">This smoothing is not a simple moving average. For firn smoothing, the gas age distributions are modeled with asymmetric filters like log-normal functions. These filters are derived from physical firn models and are backward-oriented, with variable weighting and more emphasis on relatively recent air but with a tail incorporating older mixed air.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Boron Isotopes: Lower Resolution with Age<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">The boron isotope proxy from fossil shells records ancient seawater pH, which is tightly linked to atmospheric CO\u2082 via ocean chemistry (de la Vega, 2023). Boron faithfully captures the glacial\u2013interglacial CO\u2082 cycles across the past 400,000 years. Analytical and calibration uncertainties translate roughly to 10-20 ppm at lower CO\u2082 glacial values and 15-30 ppm at higher interglacial values.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Temporal resolution depends on sedimentation rate and bioturbation. Temporal resolution is&nbsp;<strong>300 years<\/strong>&nbsp;at best during the Holocene and up to 1000 years in older deeper sediments. As sedimentation rates are lower with age, resolution decreases and bioturbation smooths the signals.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Matching the Past Records: Muted Modern CO\u2082 (MMCO\u2082)<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">The Mauna Loa CO\u2082 record, starting in 1958, provides direct measurements of atmospheric CO\u2082 for only the past 67 years. During this time, CO\u2082 rose annually from 315 ppm to 427 ppm.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">For earlier years, CO\u2082 values are from high-resolution ice core records primarily Law Dome firn and ice, which has excellent temporal resolution due to high snow accumulation. These records show CO\u2082 rising slowly from ~305 ppm around 1925 to ~315 ppm by 1958 and overlap well with Mauna Loa records.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">To simulate how modern CO\u2082 would appear in a low-resolution Antarctic ice core, 100-300 year backward smoothing filters (moving average and log-normal) were applied to the combined Law Dome and Mauna Loa data (Figure 2).<\/p>\n\n\n\n<figure class=\"wp-block-image size-large\"><img data-recalc-dims=\"1\" loading=\"lazy\" decoding=\"async\" width=\"723\" height=\"381\" data-attachment-id=\"430596\" data-permalink=\"https:\/\/climatescience.press\/?attachment_id=430596\" data-orig-file=\"https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-66.png?fit=1280%2C674&amp;ssl=1\" data-orig-size=\"1280,674\" data-comments-opened=\"1\" data-image-meta=\"{&quot;aperture&quot;:&quot;0&quot;,&quot;credit&quot;:&quot;&quot;,&quot;camera&quot;:&quot;&quot;,&quot;caption&quot;:&quot;&quot;,&quot;created_timestamp&quot;:&quot;0&quot;,&quot;copyright&quot;:&quot;&quot;,&quot;focal_length&quot;:&quot;0&quot;,&quot;iso&quot;:&quot;0&quot;,&quot;shutter_speed&quot;:&quot;0&quot;,&quot;title&quot;:&quot;&quot;,&quot;orientation&quot;:&quot;0&quot;}\" data-image-title=\"image\" data-image-description=\"\" data-image-caption=\"\" data-large-file=\"https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-66.png?fit=723%2C381&amp;ssl=1\" src=\"https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-66.png?resize=723%2C381&#038;ssl=1\" alt=\"\" class=\"wp-image-430596\" srcset=\"https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-66.png?resize=1024%2C539&amp;ssl=1 1024w, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-66.png?resize=300%2C158&amp;ssl=1 300w, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-66.png?resize=768%2C404&amp;ssl=1 768w, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-66.png?resize=640%2C337&amp;ssl=1 640w, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-66.png?resize=1200%2C632&amp;ssl=1 1200w, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-66.png?w=1280&amp;ssl=1 1280w\" sizes=\"auto, (max-width: 723px) 100vw, 723px\" \/><figcaption class=\"wp-element-caption\">Figure 2: Firn smoothing effects for low snow accumulation sites; Dome C and Vostok. Six different calculations were run for 100, 200, and 300 years temporal resolution smoothing. Bk=backward. MMCO\u2082 = muted modern CO\u2082<\/figcaption><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">These \u201csuppressed\u201d curves illustrate how dramatically firn diffusion would dampen today\u2019s rapid rise if it were recorded in low-accumulation Antarctic ice cores. Averaged computations show the mean Muted Modern CO\u2082 (MMCO\u2082) value for 2025 is 345 ppm +24 ppm compared to the Mauna Loa annual record of 427 ppm.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">The Last Time CO\u2082 was This High<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">The smooth or muted modern CO\u2082 (~345 ppm) can now be compared to key paleo records from ice cores and boron with similar resolution over the past 3 million years (figure 3).<\/p>\n\n\n\n<figure class=\"wp-block-image size-large\"><img data-recalc-dims=\"1\" loading=\"lazy\" decoding=\"async\" width=\"723\" height=\"381\" data-attachment-id=\"430598\" data-permalink=\"https:\/\/climatescience.press\/?attachment_id=430598\" data-orig-file=\"https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-67.png?fit=1273%2C670&amp;ssl=1\" data-orig-size=\"1273,670\" data-comments-opened=\"1\" data-image-meta=\"{&quot;aperture&quot;:&quot;0&quot;,&quot;credit&quot;:&quot;&quot;,&quot;camera&quot;:&quot;&quot;,&quot;caption&quot;:&quot;&quot;,&quot;created_timestamp&quot;:&quot;0&quot;,&quot;copyright&quot;:&quot;&quot;,&quot;focal_length&quot;:&quot;0&quot;,&quot;iso&quot;:&quot;0&quot;,&quot;shutter_speed&quot;:&quot;0&quot;,&quot;title&quot;:&quot;&quot;,&quot;orientation&quot;:&quot;0&quot;}\" data-image-title=\"image\" data-image-description=\"\" data-image-caption=\"\" data-large-file=\"https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-67.png?fit=723%2C381&amp;ssl=1\" src=\"https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-67.png?resize=723%2C381&#038;ssl=1\" alt=\"\" class=\"wp-image-430598\" srcset=\"https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-67.png?resize=1024%2C539&amp;ssl=1 1024w, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-67.png?resize=300%2C158&amp;ssl=1 300w, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-67.png?resize=768%2C404&amp;ssl=1 768w, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-67.png?resize=640%2C337&amp;ssl=1 640w, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-67.png?resize=1200%2C632&amp;ssl=1 1200w, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/image-67.png?w=1273&amp;ssl=1 1273w\" sizes=\"auto, (max-width: 723px) 100vw, 723px\" \/><figcaption class=\"wp-element-caption\"><em>Figure 3. Dampened modern CO\u2082 (MMCO\u2082) levels in red compared to past proxy data. References in legend.<\/em><\/figcaption><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">Key observations emerge:<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Ice core maxima over the past 800,000 years peak near 300 ppm, below the MMCO\u2082 of 345 ppm. Even the lower end of the smoothed range (~320 ppm) exceeds the interglacial peaks.<\/li>\n\n\n\n<li>Some boron isotope reconstructions approach MMCO\u2082 values during MIS 5 (~122,000 years) and MIS 9 (~330,000 years), though uncertainties are larger.<\/li>\n\n\n\n<li>Early Pleistocene (~1.1 and 1.4 my) boron estimates approach the MMCO\u2082 mean value and occasionally exceed the lower end.<\/li>\n\n\n\n<li>Around 2.0 my, boron isotopes approach and exceed the MMCO\u2082 values. Additionally, blue ice values are close to the MMCO\u2082 mean (332 ppm vs 345 ppm).<\/li>\n<\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">Thus, on a 100 to 300-year resolution basis, modern CO\u2082 is likely higher than at any time in the past 800,000 years, and possibly higher than most of the past 2 million years \u2013 even after accounting for smoothing.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">But it is also true that the smoothed value (~345 ppm) lies much closer to past interglacial highs than the raw 427 ppm record suggests.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Both statements can be true.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Wrap-Up<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">The smoothed CO\u2082 value of ~345 ppm starkly contrasts with today\u2019s unsmoothed levels of 427 ppm, demonstrating how firn diffusion in ice cores would heavily dampen the modern CO\u2082 rise. When fairly compared at proxy resolution, modern CO\u2082 is only slightly higher than past interglacial peaks and lies within the uncertainty range over the past 1-2 million years.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Modern comparisons to paleo records should respect the physics and resolution limits of the proxy archives.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">References<\/h3>\n\n\n\n<p class=\"wp-block-paragraph\">Bereiter et al., (2015). Revision of the EPICA Dome C CO\u2082 record from 800 to 600 kyr before present,&nbsp;<em>Geophysical Research Letters<\/em>&nbsp;42:542-549, doi: 10.1002\/2014GL061957.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><a href=\"https:\/\/www.ncei.noaa.gov\/pub\/data\/paleo\/icecore\/antarctica\/antarctica2015co2.xls\">https:\/\/www.ncei.noaa.gov\/pub\/data\/paleo\/icecore\/antarctica\/antarctica2015CO\u2082 .xls<\/a><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">de la Vega, E., Chalk, T. B., et al. (2023). Orbital CO\u2082 reconstruction using boron isotopes during the late Pleistocene, an assessment of accuracy,&nbsp;<em>Clim. Past<\/em>, 19, 2493\u20132510,&nbsp;<a href=\"https:\/\/doi.org\/10.5194\/cp-19-2493-2023\">https:\/\/doi.org\/10.5194\/cp-19-2493-2023<\/a>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Rae. J. W., et al. (2021). Atmospheric CO\u2082 over the Past 66 Million Years from Marine Archives.&nbsp;<em>Annual Review Earth and Planetary Sciences<\/em>. 49:609-641.&nbsp;<a href=\"https:\/\/doi.org\/10.1146\/annurev-earth-082420-063026\">https:\/\/doi.org\/10.1146\/annurev-earth-082420-063026<\/a>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Yan, Y., Bender, et al. (2019). Two-million-year-old snapshots of atmospheric gases from Antarctic ice.&nbsp;<em>Nature<\/em>, 574(7780), 663\u2013666.&nbsp;<a href=\"https:\/\/doi.org\/10.1038\/s41586-019-1692-3\">https:\/\/doi.org\/10.1038\/s41586-019-1692-3<\/a>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><a href=\"https:\/\/www.science.org\/doi\/10.1126\/science.aay8178\">Nehrbass-Ahles<\/a>, C., Shin, et al. (2020). Abrupt CO\u2082 release to the atmosphere under glacial and early interglacial climate conditions.&nbsp;<em>Science<\/em>&nbsp;369(6506), 1000\u20131005.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><a href=\"https:\/\/cp.copernicus.org\/articles\/7\/473\/2011\/cp-7-473-2011.pdf\">K\u00f6hler, P.<\/a>, Knorr, G., Buiron, D., Lourantou, A., and Chappellaz, J. (2011). Abrupt rise in atmospheric CO\u2082 at the onset of the B\u00f8lling\/Aller\u00f8d: in-situ ice core data versus true atmospheric signals,&nbsp;<em>Clim. Past<\/em>, 7, 473\u2013486, <a href=\"https:\/\/doi.org\/10.5194\/cp-7-473-2011\" rel=\"nofollow\">https:\/\/doi.org\/10.5194\/cp-7-473-2011<\/a>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><\/p>\n","protected":false},"excerpt":{"rendered":"<p>Directly splicing the modern Mauna Loa record (~427 ppm in 2025) onto Antarctic ice-core data creates a visually alarming \u201chockey-stick\u201d spike. But this comparison is apples-to-oranges because ice-core proxies (especially from low-accumulation sites like Dome C or Vostok) heavily smooth atmospheric signals over 100\u2013300+ years due to firn diffusion. Rapid modern changes get \u201cmuted.\u201d<\/p>\n","protected":false},"author":121246920,"featured_media":430590,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_coblocks_attr":"","_coblocks_dimensions":"","_coblocks_responsive_height":"","_coblocks_accordion_ie_support":"","advanced_seo_description":"","jetpack_seo_html_title":"","jetpack_seo_noindex":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2},"jetpack_post_was_ever_published":false},"categories":[1],"tags":[691841848,691829997,691841849,691823878,691841846,691841845,691841847],"class_list":["post-430588","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-uncategorized","tag-boron-isotopes","tag-carbon-dioxide-co","tag-co-cycles","tag-mauna-loa-observatory","tag-modern-co","tag-proxy-co","tag-proxy-smoothing","fallback-thumbnail"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2026\/03\/0-The-Modern-CO%E2%82%82-Spike-Looks-Scarier-Than-It-Really-Is.jpg?fit=784%2C1168&ssl=1","jetpack_likes_enabled":true,"jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/paxLW1-1O0Y","jetpack-related-posts":[{"id":283636,"url":"https:\/\/climatescience.press\/?p=283636","url_meta":{"origin":430588,"position":0},"title":"97,404 Direct CO2 Measurements From 1826-2008 Indicate Humans Do Not Drive CO2 Change","author":"uwe.roland.gross","date":"10\/17\/2023","format":false,"excerpt":"Mauna Loa Observatory So the warming in recent decades may be natural. The CO\u2082 increases resulting from the natural warming may be substantially natural too. And these results are observationally supportable. Mauna Loa From NoTricksZone By\u00a0Kenneth Richard\u00a0on\u00a017. October 2023 Carbon dioxide readings at the Mauna Loa Observatory in Hawaii have\u2026","rel":"","context":"In \"CO2\"","block_context":{"text":"CO2","link":"https:\/\/climatescience.press\/?tag=co2"},"img":{"alt_text":"","src":"https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2023\/10\/0keeling-curve-1.jpg?fit=1050%2C490&ssl=1&resize=350%2C200","width":350,"height":200,"srcset":"https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2023\/10\/0keeling-curve-1.jpg?fit=1050%2C490&ssl=1&resize=350%2C200 1x, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2023\/10\/0keeling-curve-1.jpg?fit=1050%2C490&ssl=1&resize=525%2C300 1.5x, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2023\/10\/0keeling-curve-1.jpg?fit=1050%2C490&ssl=1&resize=700%2C400 2x, https:\/\/i0.wp.com\/climatescience.press\/wp-content\/uploads\/2023\/10\/0keeling-curve-1.jpg?fit=1050%2C490&ssl=1&resize=1050%2C600 3x"},"classes":[]},{"id":444389,"url":"https:\/\/climatescience.press\/?p=444389","url_meta":{"origin":430588,"position":1},"title":"Mauna Loa Hits Record 432 ppm CO\u2082 (0.04% of Atmosphere) as Oceans Continue Major Carbon Uptake \u2013 But Show Clear Limits","author":"uwe.roland.gross","date":"05\/15\/2026","format":false,"excerpt":"The famous Mauna Loa Observatory in Hawaii (NOAA data) recorded an April 2026 monthly average of 431.12 ppm CO\u2082 \u2014 a new record for that month (up from 429.64 ppm in April 2025). 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